Spray an 8 x8-inch baking pan with non-stick spray. Set aside.
In a large bowl, mix together the cake mix and powdered sugar. Add milk and butter but do not mix. Microwave for 2 minutes and immediately mix everything together until the butter is fully melted. Fold in white chocolate and sprinkles. Be careful not to over mix as the sprinkles tend to bleed their color.
Spoon into baking pan, making sure it is level and smooth on the top. Chill in the refrigerator for at least 2 hours. Cut into squares and store leftovers in fridge.