Preheat your oven to 300°F (150°C). Line a large rimmed baking sheet with parchment paper or a silicone baking mat to prevent sticking and make cleanup easier. In a very large mixing bowl, combine the Wheat Chex, Rice Chex, pretzels, and pecans. Toss them gently so you don’t break the cereal pieces.
Place the butter in a medium saucepan over medium heat. Melt the butter, then continue cooking it, whisking constantly. The butter will foam up and then develop little brown bits at the bottom. Once it smells nutty and the bits are a golden brown color, immediately remove it from the heat so it doesn’t burn.
Into the hot browned butter, whisk in the light brown sugar, pumpkin puree, pumpkin pie spice, vanilla extract, and salt. Whisk vigorously until the sugar has mostly dissolved and the mixture is smooth and emulsified. The pumpkin puree will make the mixture look thick and rich.
Pour the warm pumpkin butter mixture over the cereal and nut mixture in the large bowl. Use a large rubber spatula to gently fold the mix until every piece of cereal and every pretzel is thoroughly and evenly coated with the glaze. Take your time to ensure no dry spots remain at the bottom of the bowl.
Spread the mixture out in an even layer on your prepared baking sheet. Bake for 45 minutes, stirring gently every 15 minutes to ensure even drying and to prevent burning. Remove from the oven and let the mix cool completely on the pan. It will become much crunchier as it cools.