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Peanut Butter Brownie Fluff Cookie Sandwiches

Thick, soft, and peanut buttery brownie-like cookies filled with a fluffy cloud of marshmallow buttercream frosting! And guess what? They're gluten-free!
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Prep Time 2 hours
Cook Time 10 minutes
Total Time 2 hours 10 minutes
Course Dessert
Cuisine American
Servings 18 Sandwiches

Ingredients
  

For the Marshmallow Buttercream

  • 1 ½ cup mini marshmallows
  • ½ cup milk
  • ½ cup unsalted butter at room temperature
  • 1 teaspoon vanilla extract
  • ⅛ teaspoon almond extract (optional)
  • Pinch of salt
  • 1 cup powdered sugar

For the Brownie Cookies

  • 2 eggs
  • 2 cups peanut butter*
  • ⅔ cup brown sugar packed
  • ½ cup unsweetened cocoa powder
  • 2 teaspoons baking soda
  • 1 teaspoon vanilla extract

Instructions
 

To make the Marshmallow Buttercream

  • In a medium saucepan over medium-low heat, combine the marshmallows and milk, and stir until smooth and melted. Set aside to cool completely.
  • In the bowl of a stand mixer fitted with a paddle attachment, cream the butter until light and fluffy.
  • With the mixer on low, slowly pour in the marshmallow mixture, followed by the vanilla, almond extract, and salt.
  • While continuing to whip the frosting, gradually add the powdered sugar and whip until light and creamy. Allow buttercream to firm up before using.*

To make the Brownie Cookies

  • Preheat oven to 350ºF and line two baking sheets with parchment paper or spray with nonstick cooking spray.
  • In the bowl of a stand mixer fitted with a paddle attachment, beat the egg and peanut butter together until combined. Add the brown sugar, cocoa powder, baking soda, and vanilla, and continue mixing until fully incorporated.
  • Form dough into one-inch balls and place onto prepared baking sheet, about two-inches apart. Slightly flatten the dough balls by gently pressing down on each one.
  • Bake for 8-10 minutes. Cookies might look slightly undercooked, but they will firm up as they cool. Allow the cookies to cool on baking sheet for 10 minutes before transferring to a wire rack to cool completely.
  • To assemble the cookies, pipe the buttercream on one side of the cookie. Place a second cookie on top and gently press down.

Notes

*I wouldn't recommend using really oily peanut butter brands, which can add too much liquid to the recipe. I used Skippy, but I'm sure Jif would also work as well!
*As buttercream sits, it will firm up. Mine thickened up enough to pipe at about 3 hours.