Heat the oven to 350 degrees and line a muffin tin with four paper liners.
In a medium bowl, whisk together the flour, granulated sugar, 1/4 cup cocoa powder, baking soda, and salt.
Add the milk, vegetable oil, egg, and vanilla extract to the dry ingredients. Stir until the batter is smooth and no lumps remain.
Divide the batter evenly among the four liners and bake for 18 to 20 minutes, or until a toothpick comes out clean. Let the cupcakes cool completely on a wire rack.
While the cupcakes cool, beat the powdered sugar, softened butter, and remaining 2 tablespoons cocoa powder in a small bowl until the frosting is creamy and spreadable.
Split each chocolate sandwich cookie in half to create 16 wing pieces. Spread a layer of frosting on each cooled cupcake, then press four cookie halves into the frosting on each cupcake to form the bat wings.