In a large pan melt the butter over medium heat. Add in the marshmallows and stir until melted. Remove from heat and add the mini marshmallows. Stir, but don’t let them melt all the way. Stir in rice krispies.
Place rice krispies in pan and press down with a buttered spatula. Add sprinkles if desired. Butter your knife and cut into squares. Cover with plastic wrap until ready to serve.