For the Whoopie Pies:

Preheat oven to 375 degrees F. If you don't have a whoopie pie pan, line two large baking sheets with silicone baking mats or parchment paper.

Sift the flour, cocoa powder, baking soda, and salt into a medium bowl. In the bowl of an electric mixer, beat the butter and brown sugar on low speed until combined. Increase speed to medium-high and beat for 2-3 minutes, or until light and fluffy. Add the egg and vanilla and beat for another 2 minutes.

Add half of the flour mixture then half of the milk to the batter and beat on low until combined, scraping down the sides of the bowl as necessary. Add remaining milk and flour, beating until combined.

Drop 1 tablespoonful of batter onto prepared baking sheets 2-inches apart (or directly in pockets of the whoopie pie pan). Bake for 10 minutes, or until pies spring back slightly when pressed gently. Let cakes cool on pans for 5 minutes before moving to a wire rack to cool completely.

For the filling:

In the bowl of an electric mixer, beat the Marshmallow Fluff or creme and butter on low speed until combined. Increase speed to medium and beat until light and fluffy, about 2-3 minutes. Reduce speed to low and add powdered sugar and vanilla. Mix until incorporated then increase speed to high and beat until smooth and fluffy, about 2-3 minutes more. Filling can be made about 4 hours ahead of time, kept at room temperature.

To assemble:

Spread tablespoonfuls of marshmallow filling on the flat side of half the whoopie pies then sandwich with the remaining whoopie pies. Store at room temperature in an airtight container for up to 3 days.