• Preheat oven to 350 degrees.  Use parchment paper or a silpat mat to line a baking sheet. 
• Cream the butter in a stand mixer until smooth and creamy. Add brown sugar and sugar and mix until light and fluffy (about 2-3 minutes). Scrape the sides of the bowl to make sure all ingredients are blended.
• Add in the egg and vanilla and beat until combined.
• Add in flour, dark cocoa, HALF of the Oreo crumbs, baking soda, and salt. Mix until all combined.
• Scoop out about 1 TBSP of dough and roll into balls in your hand. Roll balls in the remaining Oreo crumbs. Place dough balls on prepared pan. (12 cookies to a pan) Press down dough slightly to form thick discs.
• Bake for 8-10 minutes. Allow cookies to cool on the pan for 5-10 minutes before transferring to a cooling rack. Cool cookies completely before making sandwiches.

MAKE THE MALLOW CREME FILLING

• Cream the butter, vanilla and powdered sugar until smooth and creamy. Mix in the marshmallow fluff and crushed Oreo by hand. Chill the creme for about 10 minutes to make it easier to scoop.
• Scoop about 1-2 tablespoons of the mallow creme onto one side of a cookie and top with another cookie to sandwich them together.