Line a 9x13-inch baking dish with aluminum foil or wax paper, leaving a 2-inch overhang on each of the longer sides. Spray the foil or paper lightly with cooking spray.
Melt the butter in a large pot over medium heat. Add the marshmallows and cook, stirring occasionally, until melted and smooth, about 5 minutes. Remove from the heat and stir in the salt, if desired.
Working quickly, add the rice cereal to the pot and stir with a rubber spatula until evenly coated. Transfer to the prepared baking dish and press into an even, compact layer. (Spray your hands with a little cooking spray to keep them from sticking when pressing the cereal mixture into the pan). Decorate with toppings if using (see Notes).
Let sit at room temperature until firm, about 30 minutes. Cut into 12-24 squares. Store the cereal treats at room temperature in an airtight container for up to 3 days.
Notes
To decorate these cereal treats for holidays and special occasions, sprinkle the top with 1/4 cup of your favorite sprinkles, chocolate chips and/or candy-coated chocolates (such as M&M’s) while still warm, pressing gently so they adhere.