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1
For the cakes:
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Adjust oven racks to upper-middle and lower-middle positions and heat oven to 350 degrees. Line 2 baking sheets with parchment paper.
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3
Whisk flour, cocoa powder, baking soda, and salt in medium bowl.
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4
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.
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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.
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6
Using 1/3-cup measure, scoop 6 mounds of batter onto each baking sheet, spacing mounds about 3 inches apart.
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Bake until cakes spring back when pressed, 15 to 18 minutes, switching and rotating pans halfway through baking.
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Cool completely on baking sheets, at least 1 hour.
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For the filling:
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With electric mixer on medium speed, beat butter and sugar together until fluffy, about 2 minutes.
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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.)
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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.
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13
Whoopie pies can be refrigerated in airtight container for up to 3 days.
