Grease a 1/2 dry measuring cup with butter and line a 12-cup muffin tray with cupcake liners. Or grease the inside of the cups as well if you're not using liners.
Melt the butter in a large pot over medium heat. Add all but 1 cup of the marshmallows and cook, stirring occasionally, until the marshmallows melt completely, about 5 minutes. Remove from the heat and stir in the vanilla and a pinch of salt. Add the cereal and stir with a rubber spatula, making sure to scrape up the bottom, until the cereal is completely coated. Let the filling cool off for a few minutes and then quickly fold in the reserved 1 cup marshmallows and the chocolate bars.
Use the prepared measuring cup to scoop the mixture into the cups of the prepared muffin tray. Pat the domes together gently to help the treats hold their shape. Let sit at room temperature until firm, about 30 minutes.
Store at room temperature in an airtight container for up to 3 days.
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