Whoopie Pies
Cakes:
- 1 cup (2 sticks) butter, softened
- 2 cups brown sugar, packed
- 2 tsp. salt
- 1 Tbsp. pure vanilla extract
- 2 eggs
- 3½ cups all-purpose flour
- 1 cup natural cocoa powder
- 2 Tbsp. baking soda
- 2 cups buttermilk
Filling:
- 6 Tbsp. flour
- 1/8 tsp. salt
- 2 cups milk
- 1½ cups (3 sticks) butter, softened
- 3 cups confectioners sugar
- 1 Tbsp. vanilla extract
Make the cakes: Preheat the oven to 350 degrees F and line cookie sheets with parchment paper. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, sugar, and salt until fluffy, about 2 minutes. Add the vanilla extract. Add the eggs, one at a time, and beat until combined, scraping down the sides of the bowl when necessary. In a medium bowl, whisk together the flour, cocoa powder, and baking soda. Add one-third of the flour mixture, then half the buttermilk, then another third of the flour mixture, then the rest of the buttermilk. Scrape down the sides of the bowl. Add the rest of the flour mixture and mix just until combined.
Scoop the batter using a 1½-tablespoon scoop onto the prepared baking sheets, leaving at least 2 inches in between each cake. Bake for about 10 minutes, or until just spring back to the touch and look dry on the top. Let cool on cookie sheets for about 3 minutes, then use a metal scraper/lifter to lift them off the sheets and onto a cooling rack to cool completely.
Make the filling: In a medium saucepan, whisk together the flour, salt, and milk until smooth. Cook over medium-high heat until thickened, about 5 to 7 minutes. Let cool for 2 minutes, then cover and refrigerate until completely cooled.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter, sugar, and vanilla until fluffy. Add the cooled milk mixture and beat until fluffy and resembles whipped cream, about 7 minutes. Place filling in a pastry bag and pipe onto half of cakes, or just spread onto half of cakes with a knife. Place another cake on top of each cake with filling and press down slightly. Store in an airtight container with parchment or wax paper between the layers (they will stick together).