Line rimmed sheet pan with parchment paper or wax paper, or butter it well. 

In a large pot, melt butter over medium-low heat. It will melt, then foam, then turn clear golden and finally start to turn brown and smell nutty. Watch closely and stir often. 

When butter is evenly browned, stir in marshmallows. (If using unsalted butter, stir in 1/8 teaspoon salt.) Melt and cook, stirring often, until mixture turns pale brown, then stir constantly until lightly browned but not dark, 3 to 5 minutes. 

Turn off heat, add cereal, and mix well, preferably with a silicone spoon or a spatula. Scrape into prepared pan and press down lightly. If necessary, butter hands to press mixture flat. Let cool, and cut into squares or bars.