Prep your pan: Line an 8x8-inch pan with parchment paper, leaving overhang for easy lifting. Lightly mist with cooking spray.
Mix the dry base: In a large bowl, add the crispy rice cereal.
If using coconut, nuts, or chocolate chips, set them aside for now.
Heat the binder: In a medium saucepan over low heat, add butter or coconut oil, almond butter, marshmallows, and sugar-free syrup. Stir constantly until the mixture is smooth and glossy. Don’t let it bubble hard—gentle heat keeps it soft.
Add flavor: Remove from heat.
Stir in vanilla extract and a pinch of salt.
Incorporate protein: Whisk the protein powder into the warm mixture a little at a time. Stir until fully combined and silky. If it gets too thick, add 1–2 teaspoons warm water to loosen.
Combine with cereal: Pour the warm protein-marshmallow mixture over the cereal.
Using a silicone spatula, fold until the cereal is evenly coated. Work quickly—the mix firms up as it cools.
Add mix-ins (optional): Fold in coconut, nuts, or chocolate chips. If using chips, let the mixture cool 1–2 minutes so they don’t melt completely.
Press into pan: Transfer to the prepared pan.
Lightly coat your hands or a spatula with cooking spray and press the mixture evenly into the corners. Don’t smash too hard or you’ll lose the airy crunch.
Set and slice: Chill in the fridge for 30–45 minutes until firm. Lift out with the parchment and slice into 12 bars or 16 small squares.