Melt 6 tablespoons of salted butter in a medium saucepan over low heat. Add 10 oz of marshmallows and, if using, 2-3 tablespoons of marshmallow creme. Stir constantly for 5 minutes until smooth.

Remove from heat and add ⅔ cup of dry cake mix, stirring until fully combined.

In a large mixing bowl, combine 6 cups of crispy rice cereal with the marshmallow mixture. Stir in ½ to ¾ of the rainbow sprinkles.

Grease a 9x13-inch baking dish with a little butter. Transfer the mixture into the dish, spreading it evenly and pressing down firmly.

Sprinkle the remaining rainbow sprinkles on top and allow to cool at room temperature for about 30 minutes.

Once cooled, cut into squares and serve.