Make the pies: Heat oven to 350° F with the racks in the upper and lower thirds. Line 2 large baking sheets with parchment.
Whisk together the flour, cocoa, baking soda, salt, and baking powder; set aside.
Beat the butter and brown sugar in a large bowl with an electric mixer on medium-high until light and fluffy, 2 to 3 minutes. Beat in the egg, then the vanilla, scraping down the sides of the bowl as necessary.
Reduce mixer speed to low. Add the flour mixture and buttermilk in alternate additions (scraping down the sides of the bowl as necessary), beginning and ending with the flour mixture. Mix until combined.
Spoon the batter into 24 mounds (about 3 tablespoons each) on the prepared baking sheets, 2 inches apart.
Bake, rotating the sheets once, until the pies spring back lightly when touched, 10 to 14 minutes. Let cool slightly, then transfer to a wire rack to cool completely.
Make the frosting: Beat the cream cheese, heavy cream, butter, and salt in a large bowl with an electric mixer on high until light and fluffy, 2 to 3 minutes. Reduce mixer speed to low and gradually add the confectioners’ sugar, then the vanilla; beat until smooth, scraping down the sides of the bowl as necessary. Chill in the refrigerator until almost firm, at least 30 minutes.
Top half the pies with the frosting, dividing evenly, and sandwich with the remaining pies.
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