Preheat oven to 350℉. Spray a 9x9 inch glass pan with non-stick cooking spray.
Combine the cake mix, oil, and egg in a large bowl.
Add the milk to the bowl slowly, you want the batter to remain as dense as possible.
Fold in the rainbow sprinkles and white chocolate chips.
Pour batter in pan and spread to the edges.
Bake for 25-30 minutes (they will be a little soft/gooey in the middle- just let them sit for about 20-30 minutes and they will set up).
Cool completely.
To make frosting, combine all frosting ingredients together in a bowl and frost bars.
If frosting is too thick, add a little more milk.
Notes
Yellow cake mix can be used instead of funfetti cake mix. You will need to add 1/4 cup of sprinkles to the mix. (frosting is optional because these taste amazing without frosting, but I am a serious frosting lover, so I added some and loved the way it turned out).