Lightly butter a large heatproof bowl, add cereal and set aside. Lightly butter a 13x9" baking dish, then line with parchment paper, leaving a 2" overhang on long sides. Lightly butter parchment. Cook 3/4 cup butter in a large skillet over medium heat, swirling, until butter foams, then browns (be careful not to let it burn), 5–7 minutes. Add marshmallows and salt. Reduce heat to low and cook, stirring constantly, until marshmallows are melted and mixture is smooth, about 3 minutes. Mixture will have brown specks and almost look grayish in color, and that’s ok!
Pour marshmallow mixture over cereal scraping the pan to get all the mixture into the bowl. Stir with a large heatproof spatula or large spoon, scraping bottom of bowl and folding, until cereal is completely coated, mixture will get very sticky and thicken as it cools so work quickly. Scrape into prepared pan; then cover with a sheet of parchment paper. Press down firmly to make an even, compact layer (just don’t press too hard or you’ll break the loops!). Cover treats loosely with plastic and let sit until cool, about 2 hours.
Just before serving, using parchment paper overhang, lift treats out of pan and place on a cutting board; remove parchment. Using a serrated knife, trim about 1/2" from all sides to make a squared-off rectangle, then cut into an 8x6 grid to make 24 squares.
Do Ahead: Treats can be made 1 day ahead. Leave in pan. Store tightly wrapped at room temperature. Cut into squares just before serving.