Start by preparing your pan and covering the bottom and sides of a 9x13x2 pan with parchment paper. You can also spray the sides and bottom of the pan with cooking spray. Combine butter and marshmallows in a microwave-safe bowl, and microwave in 30-second intervals.
You can also use a stovetop, and heat in low heat until butter melts and the marshmallow mixture is smooth, about 1-2 minutes, stirring occasionally. Add the creamy peanut butter and mix well. Add the cereal and combine until well-coated.
Press the peanut butter mixture in your prepared pan with waxed paper -to prevent it from sticking in your hands- until well distributed. Melt the chocolate and butter in the microwave, in 15 second increments until you can whisk it together.
When the melted chocolate is cool to touch, top the cereal mixture in an even layer. Cover with plastic wrap and refrigerate until chocolate hardens, about 30 minutes. Sprinkle with sea salt and cut into even squares.