Preheat your oven to 300°F (150°C). Line a large rimmed baking sheet with parchment paper or a silicone baking mat. If you don’t have one large pan, use two smaller pans so the mix can spread out in an even layer.
In a very large mixing bowl, add 8 cups Chex cereal and 2 cups small pretzels. Gently toss together with your hands or a large spoon so you have an even mix.
8 cups Chex cereal, 2 cups small pretzels
In a medium saucepan over medium heat, combine 1 cup unsalted butter, 1 cup packed light brown sugar, and 1/2 cup granulated sugar. Stir frequently until the butter is melted and the sugar is mostly dissolved into a smooth, glossy sauce. Remove from heat and stir in 2 1/2 teaspoons ground cinnamon, 1 teaspoon vanilla extract, and 1/4 teaspoon fine sea salt.
1 cup unsalted butter, 1 cup light brown sugar, 1/2 cup granulated sugar, 2 1/2 teaspoons ground cinnamon, 1 teaspoon pure vanilla extract, 1/4 teaspoon fine sea salt
Pour the warm cinnamon sugar mixture over the cereal and pretzels in the large bowl. Using a spatula or large spoon, gently but thoroughly toss, scraping from the bottom and turning the mixture until all pieces are evenly coated and no dry spots remain.
8 cups Chex cereal, 2 cups small pretzels
Spread the coated cereal and pretzel mixture onto the prepared baking sheet in an even layer. Some small clumps are fine. Bake at 300°F (150°C) for 15–20 minutes, stirring and redistributing the mix halfway through. The coating should look set and slightly drier, not wet or glossy, and it should not be browned.
While the mix is baking, combine 1/2 cup granulated sugar and 1 1/2 teaspoons ground cinnamon in a small bowl. Stir until evenly mixed.
1/2 cup granulated sugar, 1 1/2 teaspoons ground cinnamon
Remove the baking sheet from the oven. While the churro Chex mix is still warm, immediately sprinkle the cinnamon sugar mixture evenly over the top. Gently toss and turn the mix right on the baking sheet so the sparkly cinnamon sugar coats all the pieces.
1/2 cup granulated sugar, 1 1/2 teaspoons ground cinnamon
Let the churro Chex mix cool completely on the baking sheet, about 20–30 minutes, until fully crisp and no longer warm. Once cool, stir in 1 cup mini marshmallows if using.
1 cup mini marshmallows
Transfer the cooled churro Chex mix to a large serving bowl, or portion into airtight containers, jars, or treat bags for storage and gifting.