Preheat your oven to 275°F (135°C). Line a large rimmed baking sheet with parchment paper. If your sheet is small, use two pans so the mix can spread out in a thin, even layer.
In a very large mixing bowl, add the Rice Chex, Corn Chex, mini pretzels, pecan halves, and roasted pumpkin seeds. Gently toss with your hands or a large spatula until evenly mixed, trying not to crush the cereal.
3 cups Rice Chex cereal, 3 cups Corn Chex cereal, 2 cups mini pretzels, 1 cup pecan halves, 1/2 cup roasted pumpkin seeds
In a medium microwave-safe bowl or small saucepan, combine the melted butter, light brown sugar, and maple syrup (or honey). Heat in the microwave in 20–30 second bursts, or over low heat on the stove, just until the butter is melted and the sugar begins to dissolve. Whisk until smooth, glossy, and slightly thick but still pourable.
1/2 cup unsalted butter, 1/2 cup light brown sugar, 1/4 cup pure maple syrup
Whisk the vanilla extract, pumpkin pie spice, optional extra cinnamon, salt, and optional pumpkin puree into the warm mixture until completely smooth. Taste and adjust the spice if desired.
2 teaspoons pure vanilla extract, 2–3 teaspoons pumpkin pie spice, 1/2 teaspoon ground cinnamon, 1/2 teaspoon fine sea salt, 1 tablespoon pumpkin puree
Slowly pour the warm pumpkin spice glaze over the cereal mixture, scraping the bowl to get every bit. Using a large spatula, gently toss and fold from the bottom up until most pieces look lightly and evenly coated with a shiny glaze.
Transfer the coated mix to the prepared parchment-lined baking sheet. Spread into an even layer, breaking up any very large clumps but leaving some clusters. If needed, divide between two pans so it bakes evenly.
Bake at 275°F (135°C) for 45–60 minutes, stirring every 15 minutes. Each time you stir, gently flip sections of the mix and spread it back out. The mix is done when it is mostly dry to the touch, lightly golden, and smells very fragrant and toasty. If you used pumpkin puree, lean toward the full 60 minutes so it dries completely.
Remove the baking sheet from the oven and let the Chex mix cool completely on the pan; it will crisp as it cools. Once fully cool, break up any very large clusters, then stir in the fall-colored M&M’s or white baking chips, if using, so they don’t melt.
1/2–1 cup fall-colored M&M’s or white baking chips
Taste a handful and, if needed, sprinkle a tiny pinch of fine salt over the mix while it is still just slightly warm, then toss again. Transfer the cooled mix to airtight containers, jars, or treat bags for serving and gifting.