peanut butter whoopie pies

from martha stewart. fun to make and yummy to eat.

1 3/4 cups all-purpose flour
3/4 cup unsweetened cocoa powder, not dutch-process
1 1/2 teaspoons baking soda
1/2 teaspoon coarse salt
1 tablespoon unsalted butter, softened
1/4 cup vegetable shortening
1/2 cup granulated sugar
1/2 cup packed dark-brown sugar
1 large egg
1 cup whole milk
1 teaspoon pure vanilla extract


filling:
2/3 cup natural, creamy peanut butter
1/2 cup (1 stick) unsalted butter, softened
3/4 cup confectioners' sugar
Coarse salt, optional

preheat the oven to 375 degrees. sift together flour, cocoa powder, baking soda, and salt into a small bowl; set aside.
put butter, shortening, and sugars into the bowl of an electric mixer fitted with the paddle attachment. mix on high speed until smooth, about 3 minutes. add egg; mix until pale and fluffy, about 2 minutes. mix in half the flour mixture, then the milk and vanilla. mix in remaining flour mixture.

drop rounded tablespoons of dough onto baking sheets lined with parchment paper, spacing 2 inches apart. bake until cookies spring back when lightly touched, 12 to 14 minutes. let cool on sheets on wire racks 10 minutes. transfer cookies to wire racks using a spatula; let cool completely.
make filling: put peanut butter and butter in the bowl of an electric mixer fitted with the paddle attachment. mix on high speed until smooth. reduce mixer speed to low. add confectioners' sugar; mix until combined. raise speed to high, and mix until fluffy and smooth, about 3 minutes. season with salt, if desired.
assemble cookies: spread 1 scant tablespoon filling on the bottom of 1 cookie. sandwich with another cookie. repeat with remaining cookies and filling. Cookies can be refrigerated in single layers in airtight containers up to 3 days. Bring to room temperature before serving.