Combine the brown sugar, butter, corn syrup, salt, and vanilla in a microwave-safe bowl.
Microwave for 2 1/2 minutes.
Stir, then microwave for another 2 minutes.
Add the baking soda and stir carefully. The caramel will foam up.
Pour the Chex cereal and mixed nuts into a large bowl.
Pour the caramel mixture over the cereal and nuts and stir until coated.
Spread the mixture onto a cookie sheet to cool.
Once the mixture is mostly dry, pull large chunks apart into manageable pieces and let cool completely.
Notes
Use a large microwave-safe bowl because the caramel bubbles up after the baking soda is added. Stir carefully so the cereal gets evenly coated without crushing it. Let the snack mix cool before breaking it apart. Store cooled Chex mix in an airtight container at room temperature.