1 3/4 cups all- purpose flour
3/4 cup unsweetened cocoa powder
1/2 teaspoon salt
1 1/2 teaspoon baking soda
4 tablespoons unsalted butter, softened
1/2 cup vegetable shortening
1/2 cup granulated sugar
1/2 cup dark brown sugar
1 large egg
1 cup whole milk
1 teaspoon pure vanilla extract
Peanut Butter Buttercream
2/3 cup creamy peanut butter
8 tablespoons unsalted butter
3/4 cup powdered sugar
1. Preheat oven to 375F. Line two baking sheet with parchment paper; set aside. Sift together flour, cocoa, baking soda, and salt into a small bowl; set aside.
2. Add butter, shortening, and sugars to the bowl of an electric mixer fitted with the paddle attachment; cream on high speed until smooth about 3 minutes. Add egg; beat until pale and fluffy, about 2 minutes. Add half the flour mixture, then the milk and vanilla; beat until combined. Add the remaining flour mixture. Beat together.
3. Drop 12 slightly rounded tablespoons of batter 2 inches apart on baking sheets. Bake 10 minutes in upper and lower thirds of oven; switch positions and rotate. Continue baking 2 to 4 minutes or until cakes spring back when touched. Let cool on baking sheets (about 10 minutes). Transfer to wire rack to let cool completely.
Cream peanut butter and butter in medium bowl on high speed. Reduce mixer to low speed and mix in powdered sugar until combined. Beat mixture on high speed until fluffy and smooth (about 3 minutes). Add salt to taste.
Spread scant 1 tablespoon peanut butter buttercream on half of cookies. Top with remaining cakes. Repeat with remaining cookies and filling.
Cookies can be refrigerated in single layers in airtight container up to 3 days. Bring to room temperature before serving.