1. Combine the cake mix, oil, and egg in a large bowl.
2. Add the milk slowly – you want the batter to remain as dense as possible. Then mix in the rainbow sprinkles and white chocolate chips.
3. Spray pan (either 8x11 or 9x9 pan) with non-stick cooking spray.
4. Dump batter in pan and spread to the edges.
5. Bake for 25-30 minutes at 350 degrees. (They will be a little soft/gooey in the middle- just let them sit for about 20-30 minutes and they will set up.).
6. Mix frosting ingredients together and spread on brownies.