This is just incredibly decadent stuff. It's addictive. It's also about 1 million calories per cup. Be forewarned.
Basically, it's a traditional-style brown sugar caramel corn that has been whacked into something completely different through the addition of a hard-crack style pure sugar caramel. If you don't have experience making caramel, don't be afraid to try this! But - understand that caramel can be quite dangerous. Boiling sugar holds heat almost as well as oil - if you spill a pan of this on you, you've got a trip to the emergency room coming. So, if this is your first time making caramel just remember that if it looks like something is going to fall or spill let it and get out of its way. It's just sugar; you can buy more. Sugar can also burn if it gets too hot - the results taste horrible. Be prepared in case you burn your caramel! You should have something you can pour it into if you're going to need to throw it away - something that can withstand a lot of heat; say, an old coffee can or a cardboard box. I once burned a large batch and had to toss about a pound of smoking sugar out onto the back porch where it a) melted a piece of vinyl pipe it landed on and b) congealed and required a cold chisel and hammer to remove. Don't use your best pans to make this stuff.
If you have a caramel melt-down, pour it out of your pan and let it cool, then put the pan under running water. The caramel will dissolve over time. Forget trying to chip it out of your pan. Time and running water are the best ways to clean up after this recipe.
Things you'll need:
Put the popped corn in the big canning pot. Have a big (strong! carmel takes force to stir!) heat-resistant spoon to stir it with sitting nearby. I use a wooden spoon and have broken 2 so far.
Pour the cashews into the popcorn pot. Do not stir them in. This way, when you add the caramel, it'll hit the cashews first and cover them first. Yum!
Put the baking soda in a spoon near the stove, ready for use.
When you get to the carmelizing, you will not have time to read directions. Read them, think about them, and just Do It.
Put the butter, corn syrup, brown sugar, cream of tartar, and salt in the 2 gallon saucepan and melt it at medium temperature. While it's doing that, go to the next step (below)...
Put the pure white sugar and the 1/4 cup water in the 1/2 gallon saucepan and heat at high heat, stirring constantly. In this pan you are going to be making a hard-crack light brown caramel. It's heat-sensitive and touchy. Spend most of your attention on this pan. It should begin to boil rapidly. Once it begins to boil rapidly bring the other pan (with the brown sugar in it) to a rapid boil also.
Stir both pans constantly but pay more attention to the pan with the white sugar in it. The sugar will begin to carmelize and will turn a kind of light straw-color when it starts. You're almost there and you need to be particularly careful of it and keep stirring. As soon as the white sugar caramel turns about the color of light (or dark depending on your taste) oak, pour it into the saucepan with the brown sugar caramel (which presumably you have also been remembering to stir, and which should be boiling merrily as well). Stir the brown sugar caramel as you're pouring in the white sugar caramel! Be careful, when you mix the caramels the mixture will expand violently and might boil over if you don't keep stirring.
Mix/stir the combined caramels to about 260degrees / hard ball stage. It should start to take on a "boiling lava" appearance and if you pull the spoon out it'll begin to make strings as the drips come off it.
Add the baking soda to the caramel, still stirring. Be careful the caramel may expand again. It'll pretty quickly start to turn white and its texture will change. Stir a little more, and pour it into the popcorn.
Stir the popcorn madly trying to cover as much of it as possible with caramel. As the caramel hardens this will get harder and harder and you may break your spoon. Once it gets too goopy to stir anymore, dump it out into cooking/roasting pans and place in an oven at 150 degrees for an hour. Stir once or twice during baking.
Turn out onto wax paper, allow to cool, break apart, and eat.
Store in tight containers (it goes bad in 2 or 3 weeks since we don't add BHA or BHT or any of those nifty preservatives)
Extra credit: melt a of bar of dark semi-sweet chocolate and drizzle it over the caramel corn once it has cooled, but before you have broken it apart.