Prepare a 9x12 Pyrex or metal pan by coating it with butter or cooking spray.
In a very large pot over medium heat, melt the butter. Once the butter is melted, add in the marshmallows. Stir the marshmallows and butter occasionally until they melt and combine together into a smooth marshmallow goo.
Working as quickly as possible, remove the melted butter marshmallow mix from the heat, and stir the dry cake mix into it in the large pot. Then stir the rice krispies into that mixture.
Stir in ½ cup Sprinkles. Press the Rice Krispie mixture into the prepared pan, and then top with the remaining ½ cup of Sprinkles, pressing them into the top.
Cut into squares, allow them to cool to room temperature, and serve!