1 (15.25 ounce) package white cake mix (such as Duncan Hines®)
3 large egg whites
½ cup salted butter, softened
1 (8 ounce) package cream cheese, softened
1 (32 ounce) package powdered sugar
½ cup unsweetened cocoa powder
4 tablespoons whole milk, or more as needed
2 teaspoons vanilla extract
Directions
Prepare cakes: Preheat the oven to 350 degrees F (175 degrees C) with racks in middle and lower third positions.
Place devil's food cake mix, 1 cup milk, 1/3 cup melted butter, 3 large eggs, and 1 teaspoon vanilla extract in the bowl of a stand mixer. Beat at low speed, using the paddle attachment, for 1 minute until combined. Increase speed to medium and beat for 2 minutes.
Spoon batter evenly into 2 (9-inch) round nonstick cake pans, greased with baking spray; set aside.