Chocolate Peanut Butter Whoopie Pies

Preheat oven to 375°F. Line two large baking sheets with parchment paper or a silicone mat; set aside.

In a bowl of an electric mixer fitted with the paddle attachment, combine butter, shortening, and sugars. Cream on high speed until smooth, about 3 minutes. add egg and beat until pale and fluffy, about 2 minutes.

In a separate bowl, sift together flour, cocoa, baking soda, and salt; set aside.

Add half the flour mixture to the butter-sugar mixture, then add milk and vanilla. Beat until combined. Add remaining flour mixture and beat together. scrape down the sides of the bowl with a rubber spatula as needed.

Place 1-ounce scoops of batter on the prepared baking sheet, spacing them 2 inches apart. Bake in the preheated oven for 10 to 12 minutes, rotating sheets halfway through the baking period. Continue baking until the cookies spring back when touched, then continue for another 2 to 5 minutes.

Remove cookies from oven and let cool on baking sheets, around 10 minutes. Transfer to a wire rack; let cool completely.

Make the filling: place egg whites, sugar, and cream of tartar in the heatproof bowl of an electric mixer. Set the bowl over a saucepan with simmering water. whisk constantly until sugar is dissolved and whites are warm to the touch, around 3 to 4 minutes.

Transfer bowl to an electric mixer fitted with the whisk attachment. Start whisking on low speed, gradually increasing to high until stiff, glossy peaks form; around 5 to 7 minutes. add vanilla and mix until combined. add peanut butter and whip until smooth.

Transfer filling into a piping bag with a 1/4-inch round tip. use immediately.

Assemble the pies: pipe filling on the flat side of a cookie. Make a spiral starting from the center, going outward to the edge. Cover with another cookie, flat side down, and gently press together. Repeat with remaining cookies and filling. transfer pies to a tray and serve.