• Preheat oven to 350 degrees F. Spray a baking sheet with non stick cooking spray or line with parchment paper.
• In a large bowl cream butter and sugar until light and fluffy. Add in egg, cocoa powder, baking soda, salt, and vanilla. Stir until combined.
• Stir in buttermilk.
• Add flour a little a time until just combined. Do not over mix.
• Spoon ~¼ cup of batter at a time onto your prepared pan. I used an ice cream scoop. Place ~3 inches apart ,they will get bigger as they bake.
• Bake for 13-15 minutes or until a tooth pick inserted in the center comes out clean. Remove to a wire wrack to cool completely.

For the Filling

• In a large bowl, cream butter, marshmallow creme, and vanilla until smooth.
• Stir in powdered sugar a little at a time until just combined.
• Spread the flat side of half of the whoopie pies with marshmallow filling.
• Top with a second whoopie pie.