In a bowl, combine the powdered sugar and cupcake mix until blended.
Cut a stick (½ cup) of butter into several pieces and place in the bowl on top of the dry ingredients. Pour in the milk but do not stir. Microwave for 2 minutes.
Remove from microwave and stir until ingredients are mixed.
Add in the white chocolate chips and stir until the chocolate melts in to the batter.
Butter an 8x8 or similar sized baking dish.
Pour fudge batter in to the prepared dish.
Sprinkle some sprinkles over the top of the fudge to add a little extra garnish.
Refrigerate the fudge for several hours or until it sets.
Cut, serve and enjoy!