Line a 9x9-inch pan with parchment paper or lightly grease the inside of the pan with butter.
Melt the butter and marshmallows together in a medium pot over medium-low heat. Stir often until the mixture is smooth and marshmallows are completely melted.
Remove the pot from the heat, then add in the protein powder and mix together until fully combined.
Add in the rice cereal and quickly mix them into the melted marshmallow mixture.
Transfer the rice crispy treat mixture to your prepared baking dish, press it into an even layer, then let it sit at room temperature for 1-2 hours.
Remove the rice crispy treats from the pan and slice into squares then enjoy!