Instructions

• Line an 8-by-8-inch baking pan with aluminum foil (leave an overhang for easy removal) and lightly spray with cooking spray or lightly butter the foil; set pan aside.
• Place 5 1/2 cups Rice Krispies cereal in a large heatproof mixing bowl; set aside.
• Put 1/2 cup unsalted butter in a heavy-bottomed large pot or skillet over medium-low heat. Stir or swirl nearly continuously as the butter melts, foams, clears, then turns golden and begins to brown and smell nutty.
• As soon as the butter begins to brown, remove the pan from the heat and continue stirring for about 1 minute to stop carryover cooking.
• If using, stir 1/4 teaspoon salt into the browned butter.
• Add the entire 10-ounce bag of marshmallows to the pan. Let them soften for 30–60 seconds, then stir. If they do not fully melt from the residual heat, return the pan to very low heat and stir constantly just until the marshmallows are fully melted and smooth—do not overheat or brown the marshmallows.
• Pour the melted marshmallow-and-butter mixture over the 5 1/2 cups cereal. Stir gently and thoroughly until the cereal is evenly coated.
• Transfer the mixture into the prepared pan. Using a spatula or lightly greased hands, press the mixture firmly and evenly into the pan, packing it into the corners and edges.
• Let the bars set at room temperature for at least 2 hours before slicing. To speed up setting, refrigerate until firm.
• Store the treats in an airtight container at room temperature or in the refrigerator for up to 1 week, or in the freezer for up to 3 months.