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. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) 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. The batter will be slightly thick. Fold in the chopped Oreos.

Spoon the batter into the liners—fill only 2/3 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.

With a handheld or stand mixer fitted with a paddle attachment, 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 1/4 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 in the next step.

With a handheld or stand mixer fitted with a paddle attachment, 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 1/4 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.

You can swipe the frostings on with an icing knife or use a piping tip. I used Ateco 829 in some of these pictured cupcakes and Ateco 849 in others! Here is how to fill a piping bag to swirl the two frostings together: Place your preferred piping tip inside the bag (with the end snipped off so that the tip fits through). Place the bag inside a very tall cup or glass and fold the excess bag over the edges of the cup. Spoon the vanilla frosting into one side of the bag, leaving a little space on the other side. Spoon the chocolate frosting into the other side of the bag. Don’t worry, it doesn’t have to be perfectly neat. Remove the bag from the cup and push the frosting down to the tip, squeezing out any air bubbles as needed. Twist the top of the bag and proceed with piping. Garnish with crushed Oreo crumbs, if desired.

Cover leftover cupcakes tightly and store in the refrigerator for up to 3 days. I recommend a cupcake carrier for storing and transporting decorated cupcakes.