Melt butter in a large saucepan over low heat and add marshmallows. Stir until they begin to melt, then remove from heat.
Whisk in the (dry) cake mix, then stir in cereal so it is completely coated with marshmallow mixture.
Sprinkle in half of the sprinkles and stir. Press the mixture into a baking dish (I used a 9 x 13 inch dish) and top with remaining sprinkles. I melted some of the chocolate chips then spread the chocolate evenly over the rice krispy treats. Then I topped it with more chips and sprinkles. Let sit for about 30 minutes before cutting. I find spraying a knife with nonstick spray helps to cut them cleanly.