Preheat oven to 350°F.
Spray a 13x9 baking dish with nonstick spray. Set pan aside.
In a large mixing bowl make cake batter according to package directions (I use the egg whites for a pure white cake). Then fold in 1/4-1/2C. Sprinkles. Pour batter into prepared pan. Smooth out batter evenly. Add to oven and bake til golden brown and when a toothpick inserted in the center of cake comes out clean bout 45 minutes or so.