Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch cake pan with cooking spray.
Combine yellow cake mix, eggs, water, and vegetable oil in a large bowl; beat with an electric mixer on low speed until moistened, about 30 seconds. Increase speed to medium and beat until batter is smooth, about 2 minutes. Pour batter into the prepared cake pan.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 25 to 30 minutes. Let cool completely, about 25 minutes.
Combine heavy cream, 1/2 cup plus 3 tablespoons cocoa powder, and sugar in a chilled glass or metal bowl; beat with an electric mixer until soft peaks form. Spread over cooled cake.
Freeze cake until whipped cream layer is firm, about 1 hour. Sprinkle confetti candy on top.