Line a 9-by-13-inch baking dish with aluminum foil leaving a 2-inch overhang on the two short ends. Lightly coat the foil with butter.
Melt the butter in a large pot set over medium heat. Add the marshmallows and cook, stirring occasionally, until the marshmallows melt, about 5 minutes. Remove from the heat and stir in the vanilla and a pinch of salt and then the cereal and pretzels until well mixed. Transfer to the prepared baking dish and press into an even layer. Set aside at room temperature until cooled and firm, about 30 minutes.
Melt the chocolate and coconut oil together in a small microwave safe bowl for 30-second increments in the microwave, stirring, until smooth. Drizzle over the bars, cover and refrigerate until the chocolate hardens, about 15 minutes. Cut into bars.
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