Preheat the oven to 250° F and place a baking sheet in the oven to warm it up.
In a large bowl, combine the corn chex, pop corn, pretzels, pepitas and sunflower kernels. Toss gently with your hands, being careful not to break up the popcorn or chex cereal.
In a small, heavy pot, combine the rest of the ingredients and bring to a simmer over medium heat until the butter and brown sugar melt into a syrup. Simmer for about 1 minute to make sure the brown sugar is completely dissolved into a liquid form. This is your toffee, which will start to solidify very quickly as it cools.
Remove the warmed baking sheet from the oven, spill the party mix onto the sheet, and pour the toffee evenly over it. IMMEDIATELY, gently toss with a large spatula to coat the party mix as well as possible. Tip: I recommend a hard spatula similar to what you might use to flip pancakes. Scrape the spatula along the bottom of the baking sheet, under the chex mix and flip it over so that you're flipping the chex mix along with the toffee over dry chex mix, allowing the toffee to drip down to coat the mix.
Return the sheet to the oven and bake for 45 minutes, turning the chex mix over every 15 minutes to continue to coat as much of the mix as possible. It will be impossible to completely coat everything, nor do you want to unless you want a snack that's more sweet than salty, but a little a little of the toffee should be on everything.
If using unsalted popcorn, sprinkle additional salt on the mixture when you toss it every 15 minutes.
Remove from the oven and break up any large clumps while warm. If you wait until the party mix cools to do this, you won't be able to do it without breaking the chex, pretzels and popcorn.
Store in a sealed container, assuming it lasts longer than a few hours.