In a large microwave bowl or in a pan on the stove over low heat, melt the butter & marshmallows. Stir often and make sure not to over heat or burn.
Once melted, remove from heat. Stir in dry cake mix. You want to work quickly here as the mixture stiffens up as it cools, making it hard to stir.
Dump in the crispy rice cereal. I've found it is easier to stir 2 - 3 cups in at a time. Again, work quickly as the marshmallow mixture stiffens quick making it hard to combine.
I like ooey, gooey treats so I only add 7 cups of cereal. However, if you like a firmer treat, add 8 cups or more to your liking. Add sprinkles here, if you are using them.
Press into an 8x8 pan or 9" cake pan. Grease hands with a little butter or oil so you can pan them down firmly without making a total mess.
Allow treats to cool. Cut into desired serving size. I recommend anywhere from 6 - 18 treats.