Melt butter with marshmallows, 15 seconds at a time, stirring after each 15 seconds. Do this 4-5 times until marshmallows and butter are melted together and smooth.
Add 3 cups rice crispys to your marshmallows (and sprinkles if you are using them) and stir.
Using your hands, take the cereal mixture and form it into a rectangle on parchment paper. Make it as wide or thick as you like. If sticky, spray hands with cooking spray and press cereal together so it stays in a form.