Adjust oven racks to the second-from-top and second-from-bottom positions. Preheat oven to 325ºF (160ºC). Line 2 rimmed baking sheets with parchment (this will make it easier to transfer the rice krispies into a bowl after) and divide Rice Krispies evenly between the 2 sheets. Toast in the oven, stirring with a silicone spatula every 3 minutes, until evenly golden brown, about 10 minutes. Remove from oven and let cool 5 minutes. Lift up both the shorter sides of the parchment to form a “sleeve” to gather all the Rice Krispies, then transfer cereal to a large bowl. Set aside.

Meanwhile, line a 2-quart baking dish (see note) with parchment overhanging two sides and coat evenly with nonstick cooking spray. Set aside.

In a 6-quart pot, melt 4 tablespoons unsalted butter over medium heat, then cook, stirring and swirling, until milk solids turn golden brown and the butter smells nutty, 3 to 4 minutes. Reduce heat to low. Add salt and marshmallows, then, using a silicone spatula, stir until the marshmallows have just melted, about 4 minutes. Add toasted Rice Krispies and fold until evenly coated.

Scrape Rice Krispies mixture into prepared baking dish. Spray another spatula with nonstick cooking spray, then use it to press the mixture into an even layer.

Sprinkle all over with flaky salt. Allow bars to cool for 30 minutes before portioning.

To portion, remove bars from the baking dish and place on a cutting board. Using a serrated knife, cut bars into squares, rectangles, or your desired shape.