Line a 9-by-13-inch baking dish with aluminum foil leaving a 2-inch overhang on all sides. Grease 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 completely, about 5 minutes. Remove from the heat and stir in the peanut butter, vanilla and a pinch of salt. Add the cereal, pretzels and coconut and stir with a rubber spatula, making sure to scrape the bottom, until the cereal is completely coated. Transfer the mixture to the prepared baking dish and pat it into an even layer (use the bottom of a measuring cup to help). Let it sit at room temperature until firm, about 30 minutes.
Melt the chocolate and coconut oil together in a small microwave-safe bowl in 30-second increments in the microwave, stirring in between, until smooth. Drizzle the chocolate over the bars; cover and refrigerate until the chocolate hardens, about 15 minutes.
Cut into 24 bars. Store at room temperature in an airtight container for up to 3 days.