Preheat oven to 400 degrees. Sift together flour, salt, cocoa powder, baking soda, and baking powder. Set aside. Line 2 baking pans with Silpats (French nonstick baking mats) or parchment paper. In the bowl of an electric mixer fitted with the paddle attachment, cream butter and sugar until light and fluffy. Add eggs, buttermilk, and vanilla. Beat until well combined. Slowly add dry ingredients. Mix until combined.Advertisement
Using a 1-ounce ice cream scoop, place cookies onto lined baking pans, 12 per pan. Bake for 12 minutes. Remove to a rack to cool. Repeat with remaining batter.
Spread 2 tablespoons frosting onto each of 24 cookies. Sandwich together with remaining 24 cookies.