Preheat the oven to 350°F (177°C). Generously grease a 9×13-inch cake pan.

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

Using a handheld or stand mixer fitted with a whisk attachment, beat the butter on high speed until smooth and creamy, about 1 minute. Add the sugar and beat on high speed for 2 minutes until creamed together. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Scrape down the sides and up the bottom of the bowl with a silicone spatula as needed. Add the egg whites. Beat on high speed until combined, about 2 minutes. Then beat in the sour cream and vanilla extract. Scrape down the sides and up the bottom of the bowl as needed. Mixture will appear curdled and that’s ok. With the mixer on low speed, add the dry ingredients just until incorporated. With the mixer still running on low, slowly pour in the milk until combined. Do not overmix. Whisk it all by hand to make sure there are no lumps at the bottom of the bowl, then fold in the chopped Oreo pieces. Avoid over-mixing.

Pour batter into prepared cake pan. Bake for 32–35 minutes or until the cake is baked through. To test for doneness, insert a toothpick into the center of the cake. If it comes out clean, it is done. Allow cake to cool completely in the pan set on a wire rack. The cake must be completely cool before frosting.

In a large bowl using a handheld or stand mixer fitted with a whisk attachment, beat the cream cheese on medium speed until creamy, about 1 minute. Scrape down the sides and bottom of the bowl and beat again as needed until completely smooth (no lumps). Add the confectioners’ sugar and vanilla extract, and beat on medium speed until combined and completely smooth, at least 2 full minutes. Scrape down the sides and bottom of the bowl and beat again as needed.

Add a few Tablespoons of the cold heavy cream, and beat until the mixture smooths and thins out. Then, with the mixer running on low speed, pour in the remaining cream in a slow and steady stream. After all of the cream has been added, increase the speed to medium-high and whip until stiff peaks form, about 2–4 minutes.

If you’d like to pipe any frosting on the cake like I did in the photo above, set aside about 1 cup of frosting. Fold chopped Oreos into remaining frosting (or all of the frosting if you are not piping any of it). Spread onto cooled cake—it will be a thick layer of frosting. I find a small offset spatula is best for frosting 9×13-inch sheet cakes. For the pictured piping, I used a Wilton 1M piping tip and piped a zigzag design around the border.

Serve immediately or cover and set aside for a few hours before serving. Or cover and refrigerate for up to 1 day, then bring to room temperature before serving. You can definitely serve the cake cold, but many preferred it closer to room temperature. Frosting is sturdy enough at room temperature or in warmer weather for a few hours.

Cover leftover cake tightly and store in the refrigerator for up to 5 days.