Grease the inside of the Bundt pan with some butter.
Melt the butter in a large pot set over medium heat. Add the marshmallows and cinnamon 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 crispy rice cereal and wheat bran cereal and stir with a rubber spatula, making sure to scrape up the bottom, until the cereal is completely coated. Spoon the mixture into the prepared Bundt pan and pat down into an even layer (use the bottom of a measuring cup to help). Let sit at room temperature until firm, about 30 minutes.
To serve, invert the Bundt pan onto a serving platter. Whisk the sugar with 1 1/4 teaspoons water in a small bowl until a smooth, thick and pourable. Drizzle the glaze over the dessert and let it harden, about 5 minutes. Cut into slices. Store in an airtight container for up to 2 days.
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