Lightly butter a 13- by 9-inch.
Melt the butter in a large saucepan over low to medium-low heat until light brown specks start to form and the butter starts to smell nutty, 10 to 12 minutes. Add the marshmallows and stir until completely melted. Remove the pan from the heat and stir in the peanut butter until melted. Add the puffed rice cereal and peanuts. Stir until well coated.
Press the mixture evenly into the prepared baking dish. Cool completely.
Meanwhile, place the chocolate in a medium heatproof bowl. Microwave in 30 second intervals until the chocolate is melted.
Drizzle the chocolate over the cooled crispy rice treats and sprinkle with flaked sea salt if desired. Cut into 18 bars.