Preheat the oven to 250º Fahrenheit. Line two 13x18-inch rimmed baking sheets with silicone mats or parchment paper.
Divide the Chex Mix, Chocolate Chex, pretzels, popcorn, and cashews between the pans. Toss and spread evenly.
Melt the butter in a 4-quart pot over medium-high heat. Stir in the sugar and corn syrup until the sugar dissolves and the mixture boils.
Stop stirring. Brush down the sides of the pot with a wet pastry brush. Clip on a candy thermometer and cook to 252º Fahrenheit, 5 to 7 minutes.
Take the pot off the heat. Stir in the baking soda and vanilla. The mixture will foam.
Pour the hot candy evenly over both pans of snack mix.
Bake for 1 hour, stirring every 15 minutes and rotating the pans. Scrape the pan bottoms with each stir.
Cool completely on the pans, then break the mix apart.
Stir in the M&Ms. Store airtight.
