Line an 8-by-8-inch baking dish with a sheet of foil large enough to leave a 2-inch overhang on all sides. Grease the foil with 1 tablespoon of butter.
Melt the remaining 4 tablespoons butter in a large pot over medium heat. Add the marshmallows, stirring occasionally until they melt completely, about 5 minutes. Remove from the heat and stir in the vanilla and a pinch of salt.
Working quickly, add the rice cereal and candy 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. (Wet your hands to keep the mixture from sticking.) Let sit at room temperature until firm, about 30 minutes. Cut into 16 squares.
Store the treats at room temperature in an airtight container for up to 3 days.
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