In a medium saucepan over low heat, melt the salted butter. Add the entire bag of marshmallows, stirring constantly for about 5 minutes until fully melted and smooth.

Remove from heat and stir in dry cake mix until fully blended into the marshmallow mixture.

In a large mixing bowl, measure out the crispy rice cereal. Pour the melted mixture over the cereal and fold it in gently. Add ½ to ¾ of the rainbow sprinkles and mix until coated.

Lightly butter a 9x13-inch baking dish. Pour the sticky cereal mixture into the dish and press down firmly using a spatula.

Sprinkle the remaining rainbow sprinkles over the top before the mixture sets. Allow to cool at room temperature for about 30 minutes.