Preheat the oven to 350℉. Line muffin tin(s) with cupcake liners or grease with non-stick spray.

In a large bowl, whisk together the cake mix, vanilla pudding mix, baking powder, chocolate cookie crumbs, and chopped sandwich cookies.

In another bowl, whisk together the melted butter, eggs, vanilla, and buttermilk. Add to the dry ingredients and stir just until fully incorporated, being careful not to overmix.

Fill each cupcake liner half full with batter. Bake cupcakes in the preheated oven for 13-17 minutes or until a toothpick inserted into the center comes out clean. Let cool in the pan for 5 minutes before transferring to a wire rack to finish cooling completely before frosting. Bake the remaining cupcakes.

In a large bowl, cream together the room-temperature butter and vanilla using an electric handheld mixer on medium-high speed. Add the powdered sugar, 1 cup at a time, mixing on low between each addition. Add milk, starting with 2 tablespoons, and beat the frosting on high speed until fluffy. If making cookies and cream buttercream, fold in ½ cup of chocolate cookie crumbs at this point.

Frost the fully cooled cupcakes using a piping bag fitted with your desired tip. Note: I have found using a piping tip with a bigger opening works better for the cookies & cream frosting as the cookie crumbs might get stuck in a smaller piping tip.