Arrange two oven racks near the center and preheat oven to 250°F (120°C). Line two baking sheets with aluminum foil and generously coat with cooking spray. In a very large bowl, combine the Chex cereal, nuts, and pretzels; set aside.

In a large saucepan over medium heat, melt the butter. Once melted, add dark brown sugar, corn syrup, and salt. Stir frequently and bring the mixture to a boil. Let it boil for 2 minutes, then remove from heat.

Very carefully stir in the vanilla extract. Then add the baking soda and mix well. The mixture will foam up significantly—continue stirring until the baking soda is thoroughly incorporated.

Pour the hot toffee mixture over the cereal, nuts, and pretzels. Gently fold everything together, working from the bottom upward, to get an even coating. Use caution, as the toffee is very hot.

Divide the coated mixture evenly between the two prepared baking sheets and spread it out. Bake for 30 to 55 minutes, stirring every 15 minutes—bake 30 minutes for a chewier texture, or up to 55 minutes for a crispier result. You'll notice the toffee will liquefy as it bakes, helping to coat the mix.

Take the pans from the oven and immediately stir and separate the mixture so clumps don't form as it cools. Once the pieces are cool enough to stay separate, transfer to a serving bowl or storage container.