Savor these ultra-chewy oatmeal cream pie bars topped with a cloud of marshmallow buttercream. A nostalgic, easy-to-slice upgrade to the classic snack.
- Moisture Retention: The combination of melted butter and vegetable oil limits gluten development, keeping the oat base incredibly soft and moist for days.
- Starch Gelatinization: Baking at a gentle 325°F ensures the quick-cook oats hydrate and gelatinize perfectly without drying out the edges.
- Stable Emulsion: Whipping room-temperature butter with marshmallow creme creates a structurally sound, airy frosting that won't weep or slide off the bars.
I still remember peeling back the clear plastic wrapper of a Little Debbie in the middle school cafeteria, my thumb accidentally denting the soft, spiced cookie top. It was a messy, sweet ritual that always left my fingers slightly tacky. This bar version captures that exact, nostalgic magic without the fuss of baking individual cookies that inevitably spread into sad, flat puddles on your baking sheet.
These bars are a total game-changer.
Baking copycat oatmeal pies often means struggling to keep them soft without them falling apart. Shifting the dough into a 13x9 pan bypasses that structural anxiety entirely. You get the ultimate texture of thick, soft & chewy oatmeal cookies in a sturdy, sliceable format.
The secret lies in the fat profile of the dough. We use a hybrid of melted butter and vegetable oil.
This combination is pure food science. Melted butter provides rich dairy flavor and assists in the Maillard reaction for a golden crust, while the vegetable oil remains liquid at room temperature. This liquid fat prevents the flour's protein matrix from tightening up, ensuring the bars stay incredibly tender for days. It feels a bit like cake batter when you spread it, but trust the process.
Just use a sturdy offset spatula.
When it comes to the frosting, we are not making a standard, heavy buttercream. We are whipping up a cloud-like marshmallow dream.
Beating room-temperature butter with marshmallow fluff creates that iconic, slightly tacky filling. It is the same technique that makes our cookie sandwiches with no-fail marshmallow buttercream frosting so addictive. Just ensure the base is completely cool before frosting.
Oatmeal Cream Pie Bars
Ingredients
- 1/2 cup butter, melted
- 1/4 cup vegetable oil
- 1 cup brown sugar, packed
- 2 eggs
- 1 tablespoon vanilla extract
- 1.5 teaspoons cinnamon
- 1 teaspoon salt
- 1/2 teaspoon baking soda
- 1/4 teaspoon nutmeg
- 1/4 teaspoon ginger
- 1 cup flour, packed
- 1.75 cups quick-cook oats
- 1 7-oz. tub marshmallow creme
- 1 cup salted butter, at room temperature
- 1 tablespoon milk or cream
- 1 teaspoon vanilla extract
- 1 1-lb. box powdered sugar
Directions
- Preheat your oven to 325°F. Lightly grease a 13x9-inch baking pan with nonstick spray, or line it with a parchment paper sling, leaving an overhang on the long sides for easy removal later.
- In a large mixing bowl, combine the 1/2 cup of melted butter, 1/4 cup of vegetable oil, 1 cup of packed brown sugar, 2 eggs, and 1 tablespoon of vanilla extract. Beat with a hand mixer on medium speed until the mixture is smooth and fully emulsified.
- Sprinkle in the 1.5 teaspoons of cinnamon, 1 teaspoon of salt, 1/2 teaspoon of baking soda, 1/4 teaspoon of nutmeg, and 1/4 teaspoon of ginger. Mix again to distribute the spices evenly throughout the wet base.
- Turn the mixer to low speed and add the 1 cup of packed flour and 1.75 cups of quick-cook oats. Mix just until the dry ingredients are fully incorporated and no flour pockets remain.
- Scrape the batter into your prepared pan and use an offset spatula to spread it into an even layer. Bake at 325°F for 20 to 25 minutes, or until the edges are lightly golden brown and the center is set.
- Place the pan on a wire rack and let the cookie base cool completely. If you used a parchment sling, carefully lift the cooled base out of the pan before frosting; otherwise, leave it in the pan.
- To make the frosting, beat the 1 cup of room-temperature salted butter in a clean bowl until smooth and creamy. Pop the lid off the 7-ounce tub of marshmallow creme and microwave it for 15 seconds to loosen it up, then scrape it into the butter and beat until fully combined.
- Add the 1 tablespoon of milk, 1 teaspoon of vanilla extract, and the 1-pound box of powdered sugar. Mix on low speed until the sugar is incorporated, then increase to medium and whip until the frosting is light, fluffy, and easily spreadable, adding a splash more milk if it feels too stiff. Spread evenly over the cooled oat base and slice into 24 bars.
Notes
Keep these essential tips in mind to ensure your cookie bars turn out perfectly soft and delicious every single time:
- Salt Conversion: We highly recommend using Diamond Crystal kosher salt (1 teaspoon = 1/2 teaspoon of standard table salt) to balance the intense sweetness of the marshmallow creme.
- Oat Selection: Do not swap the quick-cook oats for old-fashioned rolled oats. Quick oats are smaller and pre-steamed, which allows for rapid starch gelatinization during the short bake time, keeping the bars soft instead of unpleasantly chewy.
- Workflow Warning: Keep a close eye on the oven during the final minutes. Overbaking even by two minutes will dry out the edges and ruin the soft, cake-like texture of the copycat Little Debbie experience.
- Stacking Strategy: Store the frosted bars in a single layer in an airtight container at room temperature for up to 5 days. If you must stack them, place a sheet of parchment paper between the layers to prevent the sticky marshmallow buttercream from clinging to the bars above.
- Flour Measurement: When measuring your flour, spoon it into the measuring cup and level it off rather than scooping directly from the bag, which packs the flour and leads to dry, dense bars.
Serving Suggestions
These sweet, spiced bars pair beautifully with beverages and sides that balance their rich, sugary profile:
- Cold Milk: Pair a bar with a tall glass of ice-cold whole milk to cut through the rich, sweet marshmallow frosting.
- Fresh Berries: Serve alongside tart raspberries or sliced strawberries to balance the deep, spiced brown sugar notes of the oat base.
- Hot Espresso: Enjoy with a bitter, dark roast espresso to contrast the sugary, cloud-like buttercream.
- Warm Cider: Serve during autumn with a mug of warm, spiced apple cider to complement the cinnamon, nutmeg, and ginger in the dough.
- Ice Cream Topping: Crumble a bar over a scoop of bitter dark chocolate ice cream for a fantastic play on textures.
Storage and Reheating
Unlike traditional fudgy cocoa brownies which can dry out quickly, these bars hold up surprisingly well over time, thanks to the vegetable oil in the dough which keeps the crumb moist. At room temperature, they will remain soft and delicious for up to 5 days in a tightly sealed container. However, the marshmallow buttercream is sensitive to humidity, so keep them in a cool, dry spot.
If you want to freeze them, it is best to do so before frosting. Wrap the un-iced oat slab tightly in plastic wrap and heavy-duty foil, then freeze for up to 3 months. When you are ready to serve, thaw the slab at room temperature overnight, whip up a fresh batch of marshmallow frosting, and slather it on right before slicing.
Freezing fully frosted bars is possible but comes with a slight texture loss. The marshmallow creme in the frosting can become a bit tacky and lose some of its airy volume upon thawing. If you do freeze them frosted, flash-freeze them on a baking sheet for an hour first so the buttercream solidifies, then wrap them individually.
Avoid reheating these bars in the microwave once they are frosted. The marshmallow buttercream has a low melting point and will quickly liquefy into a sweet puddle, ruining the beautiful contrast between the chewy oat base and the fluffy topping. Enjoy them at room temperature or slightly chilled from the fridge.
Ingredient Substitutes
If you need to make adjustments based on what is in your pantry, try these tested swaps:
- Vegetable Oil: Swap the 1/4 cup of vegetable oil for 1/4 cup of melted coconut oil. Ensure the coconut oil is cooled to room temperature before mixing so it doesn't prematurely cook the eggs.
- Marshmallow Creme: Substitute the 7-ounce tub of marshmallow creme with 7 ounces of mini marshmallows melted with 1 tablespoon of corn syrup over low heat. Let the mixture cool completely before whipping it into the butter to prevent the frosting from melting.
- Salted Butter (Frosting): Replace the 1 cup of salted butter with 1 cup of unsalted butter plus 1/4 teaspoon of fine sea salt. Whisk the salt into the powdered sugar first to ensure even distribution throughout the buttercream.
- Quick-Cook Oats: If you only have old-fashioned rolled oats, pulse 1.75 cups of them in a food processor 4 to 5 times until they are coarsely chopped. This mimics the smaller size of quick oats and ensures proper hydration during baking.
- Brown Sugar: Swap the 1 cup of packed brown sugar for 1 cup of granulated white sugar mixed with 1 tablespoon of molasses. Thoroughly whisk the molasses into the melted butter and oil mixture before adding the dry ingredients to prevent clumping.
There is something incredibly comforting about recreating a childhood favorite in your own kitchen. These bars deliver all the nostalgic flavor of the classic wrapped snack, but with an elevated, buttery richness that you just can't get from a box.
