Melt the butter in a saucepan over medium-low heat. Add the marshmallows. As the marshmallows melt, mix in a little cake mix at a time. After all of the marshmallows have melted and the cake mix has all been stirred in, pour over cereal. Stir well.
Press into a greased 13"x9" pan with a piece of waxed paper. Top with sprinkles, and lightly press them into the treats with the waxed paper. Cool before cutting into squares.