Place 2 racks as near to the center of the oven as possible and preheat to 250 degrees. Line 2 baking sheets with aluminum foil and spray generously with cooking spray. In a very large mixing bowl, add the entire box of cereal, assorted nuts and pretzels then set aside.
Place a large saucepan over medium heat and melt the butter. (You will need a larger than normal pan because the toffee mixture will expand by 5 times its volume once the soda is added!) When the butter is melted, add the brown sugar, corn syrup and salt. Bring the mixture to a boil, stirring frequently. Allow the mixture to boil for 2 minutes and then remove from the heat.
CAREFULLY stir in vanilla extract, then after that is mixed in, add the baking soda and mix thoroughly. The toffee will foam and bubble furiously, but just keep stirring until the baking soda is completely combined.
Pour the hot toffee over the cereal, nuts and pretzels and gently fold to combine. Be careful not to flip any toffee on yourself or get any on your fingers while stirring. Work from the bottom of the bowl bringing the ingredients up through the toffee to coat. Don’t worry about complete and even coverage; just get a relatively even distribution of toffee over the mix.
Divide the mixture between the two prepared baking sheets and spread evenly. Bake 30–55 minutes, stirring every 15 minutes—30 minutes for chewier toffee and 55 minutes for a crispier toffee. You will notice that the toffee will liquefy in the oven, making it easy to get an even coating over the mixture.
Remove from oven and continue stirring and separating the clumps until the mix is cool enough to stay separate. Transfer to a bowl and enjoy!