1-First Step: Preheat and prep your pan Preheat your oven to 350°F (177°C). Line cupcake pans with liners, and set up enough space for 16 to 17 cupcakes.

2-Second Step: Whisk dry ingredients Whisk together cake flour, baking powder, baking soda, and salt. This helps the leavening spread evenly so the cupcakes rise with a soft, even crumb.

3-Third Step: Cream butter and sugar Beat butter and granulated sugar on medium-high speed until creamed, about 3 minutes. Add egg whites and beat until smooth, about 2 minutes. Then mix in sour cream and vanilla until combined.

4-Fourth Step: Combine, then fold in cookie pieces Add the dry ingredients to the wet mixture. Slowly pour in whole milk on low speed and beat just until combined, then fold in the chopped chocolate sandwich cookies. At this point, the batter will smell like cookies and chocolate, and the crumbs will be pebbly but evenly distributed.

5-Fifth Step: Fill liners correctly Fill each liner about 2/3 full. This prevents overflow while still giving you that bakery-style dome.

6-Sixth Step: Bake Bake for 19 to 22 minutes, or until a toothpick comes out clean. Cupcake ovens vary, so start checking around the 19-minute mark.

7-Seventh Step: Cool fully Cool cupcakes in the pan for 20 minutes, then move to racks until completely cool. Frosting warm cupcakes can cause it to melt or slide, so cool is worth the wait.

8-Final Step: Make the vanilla buttercream For vanilla buttercream, beat butter until creamy. Add confectioners’ sugar, cream (heavy cream, whole milk, or half-and-half), vanilla, and salt. Beat until you reach your preferred piping or spreading consistency.

9-Final Step (continued): Make the chocolate buttercream For chocolate buttercream, beat butter until creamy. Add confectioners’ sugar, cocoa powder, cream, vanilla, and salt. Beat until smooth. Dutch-process cocoa is recommended because it deepens flavor and darkens the color.

10-Final Step (continued): Swirl and garnish To swirl, place a piping tip in the bag. Spoon vanilla frosting on one side and chocolate frosting on the other. Pipe onto cooled cupcakes, then garnish with crushed chocolate sandwich cookies if you want extra crunch.