Preheat the oven to 350 degrees Fahrenheit. Spray an 8x8-inch baking dish with cooking spray.
Mix the cake mix, sprinkles, white chocolate chips, oil, egg, and milk in a bowl until the batter is just combined.
Pour the batter into the prepared baking dish, the batter will be thick almost like cookie dough. Bake for 20 minutes or until the edges are slightly golden.
Let them cool for about 20 minutes so the gooey center can set, then serve!