Preheat oven to 350 degrees and line 36 muffin tins with cupcake liners. Lay one regular size Oreo in the bottom of each liner.
In a large mixing bowl, cream together butter, brown sugar, vanilla, and eggs.
In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.
Add flour mixture and buttermilk alternately to creamed butter mixture. Stir to combine.
Pour batter in to lined muffin tin over Oreo cookie. Only fill the cups half full.
Bake 18-22 minutes until done in the middle
Top with Whipped Marshmallow Frosting.