Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a bowl, whisk together flour, cocoa powder, baking soda, and salt.
In another bowl, beat butter and sugar until fluffy. Add egg and vanilla, and mix well.
Gradually add dry ingredients to wet ingredients, alternating with buttermilk. Mix until smooth.
Drop batter by tablespoonfuls onto prepared baking sheet. Bake for 10 minutes, or until set. Cool completely.
For the filling, beat marshmallow creme and butter until smooth. Gradually add powdered sugar and vanilla, and mix until fluffy.
Spread filling on the flat side of one whoopie pie, then top with another whoopie pie. Repeat with remaining pies.