Spray a 9x13-inch baking pan with cooking spray and then line it with parchment paper, leaving a two-inch overhang on two sides. Spray again and set aside.

In a large bowl, add the cereal and 1 cup of mini marshmallows. Set aside.

In a microwave-safe bowl, add 5 tablespoons of butter, the remaining mini marshmallows, and a pinch of salt. Microwave in 30-second intervals, stirring in between. Continue microwaving and stirring until all the marshmallows are melted and the mixture is smooth.

Working quickly, pour the marshmallow mixture over the cereal mixture and stir to evenly coat. Press the mixture into the prepared pan in an even layer. Let cool for 15 minutes.

In a microwave-safe bowl, add the caramels and heavy cream. Microwave in 30-second intervals, stirring in between. Continue microwaving and stirring until the caramels are melted and the mixture is smooth. Let cool for about 30 minutes.

Whisk in the peanut butter. Spread the mixture evenly over the cereal layer and refrigerate until firm, about 30 minutes.

In a small microwave-safe bowl, add the chocolate and the remaining 4 tablespoons of butter. Microwave in 30-second intervals, stirring in between, until smooth.

Spread the chocolate evenly over the caramel layer. Let stand for 5 minutes, then sprinkle with sea salt. Refrigerate until set, at least 30 minutes.

Lift the bars from the pan using the parchment overhangs. Using a serrated knife, cut into squares. Store in an airtight container in the refrigerator, with parchment paper between layers, for up to three days.