1Melt butter in large saucepan over medium low heat. Add marshmallows. Stir constantly until marshmallows melt.
Remove from heat. Add dry cake mix to melted marshmallow and butter mixture a little at a time. Stir after each addition until cake mix is fully incorporated.
Stir in rice krispies cereal and gently stir until cereal is fully coated.
If adding sprinkles, add half at this time and stir.
Press into a greased 9x13 pan. Top with remaining sprinkles, if desired.
Let stand for about 30 minutes. I have also done this with 1/2 cup strawberry cake mix for Valentines or 4th of July.