Spray an 8-by-8-inch baking dish with cooking spray and line with parchment paper, leaving a 2-inch hangover on at least two sides.
Melt the butter in a large saucepan over medium-low heat. Add the marshmallows and stir continuously until melted, about 5 minutes. Remove from the heat and stir in the mix-ins of your choice until coated.
Press the mixture into the prepared baking dish and let cool completely at room temperature, about 1 hour.
Use the parchment overhang to lift the treats from the dish. Cut into sixteen 2-inch square bars.
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