Preheat oven to 250 degrees.
Pop your popcorn - plain popcorn is best. No salt or butter. Pick through your popcorn and make sure your remove all unpopped kernels. An easy way to get rid of the kernels is to put in a gallon size bag and shake lightly - all the kernels will fall to the bottom of the bag. If you don't have a popcorn popper, see my simple solution for popping popcorn in the microwave at the end of this post.
Keep your popcorn warm by placing it into two 9X13 baking dishes or a deep roasting pan and placing it in the oven. The caramel corn turns out a lot better with warm popcorn so don't skip this step!
Melt butter in a medium, heavy, saucepan over medium heat. Stir in the brown sugar, corn syrup and salt and bring to a boil, stirring constantly.
Once the mixture is at a boil, stop stirring and just let 'er go for 4 minutes. (This is the easy part!)
Remove from heat and stir in the vanilla and baking soda. You will notice the caramel increases in volume - it gets very "fluffy". The consistency is thicker.
Pour the mixture in a thin steam over the popcorn. Use tongs to mix the popcorn and caramel making sure to thoroughly coat.
The caramel corn should look like this going in to the oven. Put the baking dishes into the oven and let bake for 45 minutes. Every 15 minutes, pull the dishes out and break up the caramel corn using tongs to turn the popcorn over and mixing it up. After 45 minutes, let the caramel corn cool completely before breaking into small pieces. DO NOT POP THESE BEAUTIES INTO YOUR MOUTH OR YOU WILL GET BURNED. Don't ask me how I know this - just trust me. That's it! It's really very easy because there is so much down time.