Prep the pan. Lightly grease a 9x13-inch baking pan or line it with parchment for easy lifting. Leave some overhang if using parchment.

Melt the base. In a large pot over low heat, melt the butter. Add 4 cups of mini marshmallows and stir continuously until smooth and fully melted. Take the pot off the heat.

Flavor boost. Stir in the vanilla extract and fine salt. This small step keeps the sweetness balanced and adds depth.

Mix in the cereal. Add the crispy rice cereal and fold gently with a spatula until every piece is coated. If you like extra pockets of goo, fold in the optional 1 cup of mini marshmallows now.

Press into the pan. Transfer the mixture to the prepared pan. Lightly grease your hands or a spatula, then press the mixture into an even layer. Don’t smash it too hard—gentle pressure keeps the bars tender, not tough.

Make the topping. In a microwave-safe bowl or small saucepan, gently melt the chocolate chips with the peanut butter. Stir until smooth and glossy. If using a microwave, heat in 20–30 second bursts, stirring between rounds.

Finish the bars. Pour the warm chocolate-peanut butter mixture over the cereal layer and spread evenly. Sprinkle with flaky sea salt and any crisp mix-ins you love (crushed pretzels are fantastic here).

Set and slice. Let the bars sit at room temperature for 45–60 minutes, or chill for 20–30 minutes until the top is firm. Lift out using the parchment and cut into squares with a sharp knife.