Prep the pan: Line a 9x9-inch pan with parchment or lightly coat with nonstick spray. Set aside.

Measure ahead: Pour the crispy rice cereal into a large mixing bowl. Have your protein powder and optional mix-ins ready.

Melt the base: In a large pot over low heat, melt the butter. Add the nut butter and stir until smooth.

Add marshmallows: Stir in the marshmallows and cook on low, stirring often, until just melted and glossy. Don’t crank the heat—high heat makes the treats tough.

Flavor boost: Remove from heat. Stir in vanilla and salt.

Incorporate protein: Sprinkle the protein powder over the melted mixture and stir gently until fully incorporated. If it seems too thick, add 1–2 teaspoons of warm water to loosen.

Combine with cereal: Pour the marshmallow mixture over the cereal and toss with a silicone spatula until every piece is coated. Work quickly. If using chocolate chips, let the mixture cool 1–2 minutes first so they don’t fully melt.

Press into pan: Transfer to the prepared pan. Lightly grease your hands or a spatula and press the mixture evenly without smashing it too hard. Gentle pressure keeps them soft.

Finish and set: Add a chocolate drizzle or sprinkles if you like. Let the pan sit at room temperature for 30–60 minutes to set.

Slice and serve: Cut into 12 squares for snack-size portions, or 9 for larger bars.