Mix dry cake mix and confectioners' sugar together in a large microwave safe bowl.
Add the butter and milk, and pop it in the microwave for 2 minutes.
Remove from microwave and stir really well, then add the white chocolate to the bowl and stir like mad. The heat from the mixture should melt the chocolate. If it doesn't you can pop back in the microwave for about 30 seconds.
Toss in the sprinkles and mix well.
Pour into a parchment lined square 8 inch cake pan. Smooth out. Toss more sprinkles on top.
Pop in the fridge for an hour to help it set up.
Cut with a sharp knife.
Inhale. Preferably before anyone gets home.
Serve with a big old "oops, I must have eaten it all" smile