Melt the butter in a saucepan.
Add mini marshmallows and stir until smooth consistency.
Then add protein powder, stir until combined.
Pour rice crisp cereal in. Fold until all pieces are covered with the marshmallow mixture.
(Optional) Add sprinkles in and fold until fully mixed.
Place in a parchment paper lined baking tin and spread out evenly.
Let the marshmallow squares cool for 20 minutes.
Cut into squares and enjoy!