Makes: 6 sandwich cookies
Start to finish: 1 hour
2 cups (10 ounces) all-purpose flour
1/2 cup (1 1/2 ounces) Dutch-processed cocoa powder
1 teaspoon baking soda
1/2 teaspoon salt
8 tablespoons unsalted butter, softened
1 cup packed (7 ounces) light brown sugar
1 large egg, room temperature
1 teaspoon vanilla extract
1 cup buttermilk
12 tablespoons unsalted butter, softened
1 1/4 cups (5 ounces) confectioners’ sugar
1 1/2 teaspoons vanilla extract
1/8 teaspoon salt
2 1/2 cups marshmallow creme
For the cakes
Adjust oven racks to upper-middle and lower-middle positions and heat oven to 350 F. Line two baking sheets with parchment paper. Whisk flour, cocoa, baking soda, and salt together in bowl.
Using stand mixer fitted with paddle, beat butter and sugar on medium speed until pale and fluffy, about 3 minutes. Add egg and beat until incorporated, scraping down bowl as needed. Add vanilla and mix until incorporated. Reduce speed to low and add flour mixture in 3 additions, alternating with buttermilk in 2 additions. Give batter final stir by hand to ensure that no flour pockets remain. Using 1/3-cup dry measuring cup, scoop 6 mounds of batter onto each prepared sheet, spaced about 3 inches apart. Bake until centers spring back when lightly pressed, 15 to 18 minutes, switching and rotating sheets halfway through baking. Let cakes cool completely on sheets.
For the filling
Using stand mixer fitted with paddle, beat butter and sugar on medium speed until fluffy, about 3 minutes. Beat in vanilla and salt. Add marshmallow creme and mix until combined, about 2 minutes. Refrigerate until slightly firm, about 30 minutes. (Filling can be refrigerated for up to 2 days.) Place 1/3 cup filling on bottom of half of cakes, then top with remaining cakes, pressing to spread filling to edge. Serve. (Whoopie pies can be refrigerated for up to 3 days.)
Nutrition information per serving: 923 calories; 364 calories from fat; 41 g fat (25 g saturated; 2 g trans fats); 141 mg cholesterol; 555 mg sodium; 131 g carbohydrate; 2 g fiber; 81 g sugar; 9 g protein.