Mix together the cake mix and powdered sugar in a large bowl. Add milk and butter (do not stir them in) and microwave for 2 minutes. It’s cool cause the butter starts to “cook” with the batter, my kids loved looking at it. Mixing everything together RIGHT away. The batter will be VERY thick. Fold in white chocolate + sprinkles. Stir gently. You do not want the sprinkles BLEED.
Spoon into an 8×8 inch pan (lined with parchment paper), Cool in the refrigerator for at least 1 hour (overnight is better). Cut into squares and enjoy