Chocolate-Peanut-Butter-Caramel Cereal Bars

Brush a 9-inch square baking pan with butter; line with parchment, leaving a 2-inch overhang on two sides. Melt 3 tablespoons butter in a pot over medium heat. Add marshmallows and salt; cook, stirring, until melted. Remove from heat; stir in cereal. Press mixture into prepared pan in an even layer. Let cool 15 minutes.

In a small saucepan, combine sugar and 3 tablespoons water. Cover and cook over medium heat, swirling pan a few times, until sugar has dissolved and mixture comes to a boil. Uncover and cook, undisturbed, until mixture turns amber, about 3 minutes more. Remove from heat. Carefully add cream and 2 tablespoons butter (it will bubble up); stir until smooth. Let cool completely, about 1 hour. Whisk in peanut butter. Spread mixture evenly over crust and refrigerate until firm, about 30 minutes.

Melt chocolate and remaining 3 tablespoons butter in a small heat-proof bowl set over (but not in) a pot of simmering water, stirring until smooth. Spread evenly over caramel layer. Let stand 5 minutes, then sprinkle with salt and refrigerate until set, at least 30 minutes and up to 24 hours. Lift from pan using overhangs. Using a serrated knife, cut into squares. Refrigerate in an airtight container between pieces of parchment up to 3 days.