Maine's Best Whoopie Pies

Adjust oven racks to upper-middle and lower-middle positions. Heat oven to 350 degrees. Line 2 baking sheets with parchment paper. NOTE: Be certain to use 2 baking sheets, as the cakes will spread while cooking.

Whisk flour, cocoa powder, baking soda, and salt in medium bowl.

With the electric mixer on medium speed, beat the sugar and butter until fluffy, approximately 4 minutes. Beat in the egg until incorporated, scraping down the sides of bowl as necessary, then beat in the vanilla. Reduce the speed to low and beat in one-third of the flour mixture, then half of the buttermilk. Repeat with half of the remaining flour mixture, then the remaining buttermilk, and finally the remaining flour mixture. Using a rubber spatula, give the batter a final stir.

Using a 1/3-cup measure, scoop 6 mounds of batter onto each baking sheet, spacing the mounds approximately 3 inches apart.

Bake until the cakes spring back when pressed, approximately 15 to 18 minutes, switching and rotating the pans halfway through baking. Cool the cakes completely on baking sheets, for at least 1 hour.

With the electric mixer on medium speed, beat the butter and sugar together until fluffy, approximately 2 minutes. Beat in the vanilla and salt. Beat in the Marshmallow Fluff until incorporated, for approximately 2 minutes.

Refrigerate the filling until slightly firm, approximately 45 minutes. (Bowl can be wrapped and refrigerated for up to 2 days.)

Spoon 1/3 cup of the filling on the center of the flat side of 6 cakes. Top each with the flat side of the remaining 6 cakes and gently press until the filling spreads to the edge of the cake.

*Whoopie pies can be refrigerated in airtight container for up to 3 days.