Line the bottom and sides of a 9-inch square baking pan with parchment, leaving a 1-inch overhang on all sides, then coat with nonstick cooking spray.

Melt the butter in a large saucepan over medium-low heat. Add the marshmallows and peanut butter and stir with a sturdy silicone spatula until smooth and melted, about 3 minutes.

Remove from the heat, add the cereal, pretzels, and peanuts and stir until evenly coated.

Immediately scrape the mixture into the prepared pan. Using the spatula, press the mixture to evenly cover the bottom of the pan.

Working quickly (you want the cereal mixture to still be quite warm so the chocolate will melt a bit and adhere to the the surface), sprinkle the  chocolate chips on top. Press the mixture with the chocolate into a flat, even layer in the pan.

Sprinkle with flaky sea salt.

Cool to room temperature in the pan on a rack. Lift out of the pan using the foil and cut into squares. Serve immediately or transfer to an airtight container.