Preheat oven to 250F°. In a large pot add oil, popping corn and salt over low/medium heat. Place cover on pot leaving a crack for air to escape. Shortly you'll hear sizzling and then little dings of popcorn popping and bouncing in your pot. This was an entertaining moment for me as I've never made popcorn from scratch. In a world of microwaves and bagged popcorn, how many of us make popcorn on a stove anymore? If you must remove the lid for whatever reason, slide a splatter screen between your pot and lid before removing the lid completely. Or you can simply remove the lid, like I did... having not thought of the splatter screen option until just now, and have popcorn popping out of your pot. I was concerned my popcorn would begin burning or get stuck to the pot. Thankfully only a few pieces were overcooked which were easily removed. When popping slows down, remove pot from heat and transfer popped popcorn to very large bowl. Two large bowls would be even better. Add in peanuts and set aside.
Prepare two large cookie sheats with parchment paper. It is important that you use parchment paper and not wax paper. Wax paper is not generally appropriate for use in an oven because the waxy coating can melt. Do you really want to chance wax ending up on your food? Use parchment paper.
The next step you can do in a medium saucepan or in a double boiler. If you have a double boiler, use it. If you don't, buy one but you can get by with cooking in a saucepan. I have a fear of food burning to the pans so I stick to my double boiler whenever possible. It works wonderfully cooking, melting and not burning items. If you've payed attention to the recipes I've shared, I use my double boiler a lot. I love it.
So... in a medium saucepan or a double boiler combine butter, brown sugar and corn syrup. In a saucepan bring to a boil and then reduce heat. In a double boiler, heat on high bringing the water below to a low boil. Reduce heat to a simmer for 5 minutes (in either case). Remove from heat, add in vanilla and baking soda. Stir well then immediately pour over your popcorn and peanuts. Toss and coat well.
Transfer to your prepared cookie sheets. If possible, try to limit the amount of bounds and bunching you have going on. A nice even layer is best. Pop in the oven for an hour. Toss every 15 minutes. When your hour is up, remove pans and let sit for a few minutes to cool. Don't walk away for too long, the caramel type mixture will harden fairly quick. using a metal spatula, starting at one side of the pan break up any bunches and push to the edge of the pan. Continue with this method, working your way to the other end of the pan. You can also do this by hand with latex kitchen gloves. Eat immediately, because trust me you will want to, or store in an airtight container.