Christina Tosi's Christmas Treats

Spray a 9- by 13-inch baking pan with nonstick vegetable-oil spray. In an extra-large saucepan over low heat, melt the butter. Add the marshmallows and heat, stirring constantly, until completely melted, about 2 minutes. (Add the peanut butter, if using, and stir to combine.) Remove from the heat, add the cereal and use a sturdy, long-handled wooden or metal spoon to stir until the cereal is well coated in the butter-marshmallow mixture. Add any optional mix-ins and stir to incorporate. Scrape the mixture into the prepared pan and spread evenly. Cut a piece of parchment or wax paper into a roughly 9- by 13-inch rectangle and spray with nonstick vegetable-oil spray. Place the greased parchment or wax paper on top of the treats and press the mixture into the pan until it's 1/2 to 1 inch thick. Let cool until firm, 15 to 20 minutes. Using a large, sharp knife cut the treats into approximately 2-inch squares. DO AHEAD: Treats can be made ahead and kept, in an airtight container at room temperature, for 2 to 3 days, or frozen up to 1 month.