1-First Step: Prep the pan and oven Start by preheating your oven to 350°F. Grease a 9×13-inch baking pan, then line it with parchment paper if you want easier lifting after baking. This extra step helps the Oreo Sheet Cake come out cleanly and makes slicing simpler later. If you are baking for a group, this pan size is ideal because it gives you neat squares that are easy to serve.

2-Second Step: Mix the dry ingredients In a medium bowl, whisk together the 2 and 1/2 cups cake flour, 2 teaspoons baking powder, 1/2 teaspoon baking soda, and 1 teaspoon salt. Mixing these ingredients first helps the leavening spread evenly through the batter. That means your Oreo Sheet Cake will rise better and bake more evenly from edge to center.

3-Third Step: Cream the butter and sugar In a large bowl, beat the 3/4 cup softened unsalted butter and 1 and 1/2 cups granulated sugar together until the mixture looks light and fluffy. This usually takes about 3 to 4 minutes with a hand mixer or stand mixer. Beating air into the butter helps the cake stay soft and gives it a nice, tender crumb.

4-Fourth Step: Add the egg whites and flavor Beat in the 4 large egg whites one at a time, mixing well after each addition. Then add the 3/4 cup sour cream and 1 tablespoon pure vanilla extract. The batter may look slightly curdled at this stage, but that is normal. The sour cream brings moisture, while the vanilla adds warmth that supports the cookies and cream flavor.

5-Fifth Step: Finish the batter Add the dry ingredients to the butter mixture in three parts, alternating with the 3/4 cup whole milk. Begin and end with the flour mixture. Mix on low speed just until combined so the cake stays soft. Fold in the 1 and 1/4 cups chopped chocolate sandwich cookies with a spatula, making sure they are spread throughout the batter without overmixing.

6-Sixth Step: Bake the cake Pour the batter into the prepared pan and smooth the top. Bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs. The top should look set and lightly golden. If you are using a dark pan, check a few minutes early so the cake does not overbake.

7-Seventh Step: Cool completely Let the cake cool in the pan for about 15 minutes, then transfer it to a wire rack if possible or let it cool fully in the pan. Cooling matters because warm cake can melt the whipped cream frosting. If you want to make this Oreo Sheet Cake ahead of time, this is a good point to stop, wrap it well, and frost later.

8-Eighth Step: Make the whipped cream frosting In a large mixing bowl, beat the 6 ounces softened full-fat cream cheese, 3/4 cup confectioners sugar, and 1 teaspoon pure vanilla extract until smooth. Slowly pour in the 1 and 1/2 cups cold heavy cream while beating on medium speed, then increase the speed and whip until the frosting becomes thick and fluffy. This texture gives the cake a creamy finish without feeling heavy.

9-Ninth Step: Fold in the cookies Once the frosting is thick and holds soft peaks, gently fold in the 1 cup chopped chocolate sandwich cookies. Folding by hand helps keep the whipped frosting airy. If you prefer a smoother topping, you can add fewer cookies, but the cookie pieces give the dessert its signature texture.

10-Tenth Step: Frost and garnish Spread the whipped cream frosting over the cooled cake in an even layer. Sprinkle extra chopped chocolate sandwich cookies on top if you want a more dramatic look. For a neat finish, chill the frosted Oreo Sheet Cake for 20 to 30 minutes before slicing. That helps the frosting set and makes clean squares much easier to cut.