Line the bottom, and sides, of a 9-inch square baking pan with foil, leaving a 1-inch overhang on all sides; coat with non-stick cooking spray.
In a large saucepan, over medium-low heat, melt the butter.
Add the marshmallows and stir with a sturdy silicone spatula until smooth and melted; about 3 minutes.
Remove from the heat, add the cereal and pretzels; 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, sprinkle the chopped chocolate on top; the cereal mixture should still be quite warm so the chocolate will melt a bit and adhere to the the surface.
Press the mixture, when chocolate has been added to top, 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.
