1-First Step: Preparation and Mise en Place Preheat your oven to 350°F (177°C). Grease a 9×13-inch cake pan and optionally line the bottom with parchment paper for easier removal. Make sure all refrigerated ingredients are brought to room temperature before you begin, especially the egg whites, sour cream, and milk. Measure all dry and wet ingredients so you can move smoothly through each step.

2-Second Step: Combine the Dry Ingredients Whisk together 2 and 1/2 cups (285g) cake flour, 2 teaspoons baking powder, 1/2 teaspoon baking soda, and 1 teaspoon salt in a medium bowl. Set this mixture aside while you cream the butter and sugar. Sifting is optional but helps ensure there are no lumps.

3-Third Step: Creaming and Building the Batter In a large mixing bowl, beat 3/4 cup (12 Tbsp; 170g) softened unsalted butter on high speed until creamy, about 1 minute. Add 1 and 1/2 cups (300g) granulated sugar and continue beating on high for about 2 minutes until light and well combined. Next, add the 4 large egg whites one at a time, beating on high after each addition so they incorporate fully and help build volume. Beat in 3/4 cup (180g) sour cream and 1 tablespoon pure vanilla extract. The batter may look a bit curdled at this stage; that is normal. To avoid overmixing later, switch to low speed for the next steps.

4-Fourth Step: Add Dry Ingredients and Milk With the mixer on low, add the reserved dry ingredients until just incorporated. Then, while the mixer is still on low, slowly pour in 3/4 cup (180ml) whole milk. Mixing on low prevents overdeveloping the flour and keeps the cake tender. Stop mixing as soon as the batter is uniform.

5-Fifth Step: Fold in the Cookie Pieces Gently fold in 1 and 1/4 cups chopped chocolate sandwich cookies (110g, about 10 whole cookies) using a spatula. Fold only until the chopped cookie pieces are distributed; overfolding can deflate the batter and make the crumb dense.

6-Sixth Step: Bake Pour the batter into the prepared 9×13-inch pan and smooth the top with an offset spatula. Bake at 350°F (177°C) for 32 to 35 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs. If you convert this to layer pans, divide the batter between two 9-inch rounds and bake 24 to 26 minutes; check for doneness with a toothpick.

7-Seventh Step: Cooling Remove the cake from the oven and allow it to cool completely on a wire rack in the pan. Cooling fully is important before frosting; frosting a warm cake can lead to a runny, uneven finish. This cake benefits from patience: allow up to 1-2 hours to reach room temperature.

8-Eighth Step: Make the Whipped Cream Frosting In a mixing bowl, beat 6 ounces (170g) full-fat brick cream cheese on medium-high speed until creamy, about 1 minute. Add 3/4 cup (90g) confectioners’ sugar and 1 teaspoon pure vanilla extract, and beat until smooth, about 2 minutes. On low speed, gradually add 1 and 1/2 cups (360ml) cold heavy cream. Increase the mixer speed to high and beat until thick and stiff peaks form, roughly 1 to 2 minutes. Reserve about 1 cup of the frosting if you plan to pipe a border. Fold 1 cup chopped chocolate sandwich cookies (90g, about 8 whole cookies) into the remaining frosting to create the classic cookies and cream texture. Keep the frosting chilled until ready to spread to help maintain structure.

9-Final Step: Assemble and Serve Spread the cookie-studded frosting evenly over the cooled cake. If piping, use the reserved plain frosting in a piping bag to create a decorative border. Garnish with mini or regular chocolate sandwich cookies if desired. Serve immediately, or refrigerate and bring to room temperature 30 to 60 minutes before serving for the best texture. The finished cake yields about 12 servings.