Preheat the oven to 300°F (150°C). Line a large rimmed baking sheet, or two smaller baking sheets, with parchment paper to prevent sticking and make cleanup easier.

In a very large mixing bowl, combine the Chex cereal, pretzels, and cinnamon sugar cereal (if using). Gently toss with clean hands or a large spatula until evenly mixed.

In a medium microwave-safe bowl or small saucepan, combine the melted butter, light brown sugar, granulated sugar, ground cinnamon, vanilla extract, and fine sea salt. Heat gently (in 30-second bursts in the microwave, stirring between each, or over low heat on the stovetop) just until the sugar begins to dissolve and the mixture looks glossy. Do not let it boil.

Pour the warm cinnamon-sugar-butter mixture over the cereal and pretzels. Using a wide spatula, gently fold and turn the mixture from the bottom of the bowl up, taking care not to crush the cereal, until everything is evenly coated.

Spread the coated churro Chex mixture onto the prepared baking sheet(s) in as even a layer as possible, avoiding deep piles. Bake for 20–25 minutes, stirring gently halfway through, until very fragrant and the mixture looks slightly darker and dry on the surface. If your oven runs hot, begin checking around 18 minutes to prevent scorching.

Remove the pan from the oven and let the mix cool for 5–7 minutes, until warm but not too hot to handle. Transfer the warm mix to a large clean bowl or a large zip-top bag. Add 1/2–3/4 cup powdered sugar and toss or shake gently until everything is lightly and evenly coated.

Spread the coated churro mix back out on the baking sheet in an even layer and let it cool completely, about 20–30 minutes. The mix will continue to crisp as it cools.

Once fully cool, break up any large clumps with your hands. Serve immediately, or transfer to an airtight container or zip-top bags for storage.