Line a 9-by-13-inch baking dish with a piece of parchment paper, leaving 1 inch of paper hanging over each side of the dish. This will help with removing the treats later. Spray the parchment paper with nonstick spray.
In a 6-quart Dutch oven over medium heat, melt the butter. Add the marshmallows and cook, stirring continuously, until the marshmallows are melted and the butter is incorporated, 3 to 5 minutes. Reduce the heat to low and quickly add the cereal and peanuts; stir to combine. Transfer the cereal mixture to the prepared baking dish. Lightly coat a spatula with cooking spray, then use it to spread out the cereal mixture evenly.
Put the melted chocolate in a plastic sandwich bag (or pastry bag), cut off one corner and drizzle over the top of the cereal treats. Refrigerate, uncovered, until set, 1 to 1 1/2 hours.
Remove the cereal treats using the parchment paper and cut into 24 bars.