Whoopie Pies

For the cakes: Adjust oven racks to upper-middle and lower-middle positions and heat oven to 350 degrees. Line 2 baking sheets with parchment paper. Whisk flour, cocoa powder, baking soda, and salt in medium bowl.

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

Using 1/3-cup measure, scoop 6 mounds of batter onto each baking sheet, spacing mounds about 3 inches apart. Bake until cakes spring back when pressed, 15 to 18 minutes, switching and rotating pans halfway through baking. Cool completely on baking sheets, at least 1 hour.

For the filling: With electric mixer on medium speed, beat butter and sugar together until fluffy, about 2 minutes. Beat in vanilla and salt. Beat in Fluff until incorporated, about 2 minutes. Refrigerate filling until slightly firm, about 30 minutes. (Bowl can be wrapped and refrigerated for up to 2 days.)

Dollop 1/3 cup filling on center of flat side of 6 cakes. Top with flat side of remaining 6 cakes and gently press until filling spreads to edge of cake. Serve. (Whoopie pies can be refrigerated in airtight container for up to 3 days.)