In a large saucepan, melt the butter over medium-low heat.
Add marshmallows and stir until completely melted and smooth.
Quickly stir in the protein powder until fully combined.
Add Rice Krispies (or crushed rice cakes) and mix until everything is evenly coated.
Line an 8x8-inch pan with parchment paper or lightly spray with oil. Press the mixture evenly into the pan.
Let it set at room temperature for about 20 minutes.
Slice into squares and enjoy!