Preheat the oven to 350°F (177°C). Line a 12-cup muffin pan with cupcake liners. Line a second pan (or bake in batches) with 4 or 5 liners—this recipe makes about 16–17 cupcakes. Set aside.

Whisk the cake flour, baking powder, baking soda, and salt together. Set aside.

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar on medium-high speed until creamed together, about 3 minutes. Scrape down the sides and bottom of the bowl with a silicone spatula as needed. Add the egg whites and beat on high speed until smooth and combined, about 2 minutes. Scrape down the sides of the bowl as needed. Add the sour cream and vanilla, and beat on medium-high speed until combined, about 1 minute.

Add the dry ingredients and then, with the mixer on low speed, slowly pour in the milk and beat just until combined. Do not over-mix. You may need to whisk it all by hand to make sure there are no lumps at the bottom of the bowl.

Fold in the chopped Oreos. Spoon the batter into the liners—fill only ⅔ full to avoid spilling over the sides.

Bake for 19–22 minutes, or until a toothpick inserted in the center comes out clean. Cool the cupcakes in the pan set on a cooling rack for 20 minutes, then remove them from the pan and return to the rack to cool completely before frosting.

To make the vanilla buttercream: Beat the butter on medium speed until creamy, about 2 minutes. Add the confectioners’ sugar, heavy cream, and vanilla. Beat on low speed for 30 seconds, then increase to medium-high speed and beat for 2 full minutes.

Add ¼ cup more confectioners’ sugar if frosting is too thin or another Tablespoon of cream if frosting is too thick. Taste. Add a pinch of salt if frosting is too sweet.

Transfer the vanilla frosting to another bowl; no need to clean the mixing bowl—you’ll make the chocolate buttercream in it next.

To make the chocolate buttercream: Beat the butter on medium speed until creamy, about 2 minutes. Add the confectioners’ sugar, cocoa powder, heavy cream, and vanilla. Beat on low speed for 30 seconds, then increase to high speed and beat for 1 full minute.

Add ¼ cup more confectioners’ sugar or cocoa powder if frosting is too thin or another Tablespoon of cream if frosting is too thick. Taste. Add a pinch of salt if frosting is too sweet.

Frost the cupcakes as desired and garnish with crushed Oreo crumbs if desired.

Cover leftover cupcakes tightly and store in the refrigerator for up to 3 days.