Prep the pan. Line a 9x13-inch baking pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment so nothing sticks.

Melt the butter. In a large pot over low heat, melt the butter gently. Keep the heat low to avoid scorching.

Add marshmallows. Stir in the mini marshmallows and cook on low, stirring constantly, until fully melted and smooth. If using extra marshmallows for pockets of goo, hold them back for now.

Stir in peanut butter, vanilla, and salt. Remove the pot from heat. Quickly stir in the peanut butter, vanilla extract, and salt until smooth and combined.

Fold in cereal. Add the Rice Krispies. Using a greased spatula, fold gently until the cereal is evenly coated. If you reserved extra marshmallows, fold them in now for soft bites throughout.

Press into the pan. Transfer the mixture to the prepared pan. Lightly grease your hands or a spatula and press evenly. Don’t compact it too hard—gentle pressure keeps the bars tender.

Make the chocolate topping. In a microwave-safe bowl, heat the chocolate chips with the coconut oil in 20–30 second bursts, stirring between each, until smooth. Or melt over a double boiler on the stove.

Top and set. Pour the melted chocolate over the bars and spread evenly. Sprinkle with flaky sea salt if using. Let the bars set at room temperature for about 1 hour, or chill for 20–30 minutes until the chocolate firms up.

Slice and serve. Lift the slab out using the parchment and cut into squares with a sharp knife. Wipe the knife between cuts for clean edges.