Peanut Butter Whoopie Pies

3 1/2 cups flour
1 1/2 cups unsweetened cocoa power
1 tablespoon baking soda
1 tablespoon baking powder
1 teaspoon salt
2 sticks butter at room temperature
2 cups sugar
2 eggs
2 cups milk at room temperature
2 teaspoons vanilla extract


Filling:

1 1/3 cup peanut butter
2 sticks butter at room temperature
1 1/2 cups powered sugar

Preheat oven to 400 degrees
Shift together flour, cocoa power, baking soda, baking powder, and salt. With an electric mixer, beat butter and sugar on medium speed until light and fluffy. Add eggs, milk, and vanilla. Beat until well combined. On low speed, slowly add dry ingredients, mix until combined.

Drop 1 1/2 tablespoons of batter on prepared baking sheets, spacing 2 inches apart. Bake until set, about 8 minutes, rotating halfway through. Cool completely on a wire rack.

Make filling: With an electric mixer, beat peanut butter and butter on high speed until smooth. Reduce speed to low, add powdered sugar, mix until well combined. Raise speed to high, and mix until fluffy and smooth, about 3 minutes.

Assemble cookies: Spread 1 heaping tablespoon filling on the flat side 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.