Warning...this stuff is addictive. Similar to the commercial variety called Poppycock, this is a combination of sugar, butter, popcorn and nuts. What's not to like? Be careful when working with the hot sugar, but once cool, be REALLY careful and don't eat the whole thing by yourself.
Preheat oven to 2500. Line a roasting pan with parchment paper, or be prepared to clean a sticky pan. Your choice.
Pop the popcorn and set aside in a large bowl. I actually pop all 5 ounces in my wok and then use that as the mixing bowl. You can use a hot air popper if you want. There's enough butter in the coating that you won't miss the oil.
Melt the butter over medium heat. Then add the brown sugar, the salt and the corn syrup. Stir occasionally, and, once it comes to a steady bubble, time it to cook for 5 minutes. Don't let it boil over. It's not necessary to build a huge fire under this. Just let it bubble for 5 minutes.
Take off the heat and add the baking soda and stir through. Then add all the nuts and stir them through. Now dump the whole mixture on the popcorn and mix it together, spreading it as evenly as possible, while not smashing the popcorn to bits. You'll need to work quickly here, as adding the nuts cools things down and this mixture wants to seize. If it does, spread it out as best as you can and make sure you do your mixing as it warms in the oven phase.
Spread the popcorn in the roasting pan and put in the oven for an hour. Stir it every 15 minutes to make sure everything is spreading evenly.
Spread it out onto waxed paper and let it cool...no really...let it cool. It will burn the roof of your mouth if you eat it now, no matter how much you want it. It cools fast. As if. Burned your mouth didn't you. I knew you wouldn't wait. Good though, isn't it?