Peanut Butter Whoopie Pies

INGREDIENTS

• 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 fine salt
• 4 tablespoons 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
• Peanut Butter Buttercream
• 2 ounces bittersweet chocolate, finely chopped