In a large saucepan over medium heat, add the unsalted butter. Allow it to melt completely, then continue to cook, swirling the pan occasionally. Watch closely as the butter foams and begins to turn a golden brown. You'll notice a nutty aroma—this is when you want to remove it from the heat to prevent burning.
Once the butter is browned, add the entire 10-ounce bag of marshmallows to the saucepan. Stir continuously until the marshmallows are fully melted and combined with the browned butter. This should take about 2-3 minutes.
Remove the saucepan from the heat and quickly add the 5 cups of Rice Krispies cereal. If you're using salt, sprinkle it in now. Stir gently but thoroughly until all the cereal is coated with the marshmallow mixture.
Transfer the mixture to your lined 9x13-inch baking pan. Using a spatula, press the mixture down evenly into the pan. For best results, dampen your hands or the spatula with a little water to prevent sticking.
Allow the treats to cool for at least 30 minutes before cutting them into squares. This helps them firm up, making it easier to slice and serve.
