Preheat the oven to 275°F, and line two large baking sheets with parchment paper.
Place the butter and brown sugar in a small microwave-safe bowl. Microwave until the butter melts, then whisk until smooth. Carefully stir in the vanilla extract.
¾ cup unsalted butter, ¾ cup light brown sugar, 2 tablespoons vanilla extract
Combine the Corn Chex, Rice Chex, and mini pretzels in a very large mixing bowl.