I make the popcorn in an air popper. I use two bowls. The first bowl is used for catching the popcorn; once the popcorn is finished popping the first time. I stop the machine. I shake the bowl so all the unpopped seeds fall to the bottom. I than scope up the popcorn from the 1st smaller bowl in to the 2nd larger bowl. I do this 2 more times with the air popper.
I than melt the butter in a large pan on the stove top; once butter is almost melted; I add the brown sugar, karo syrup, and the salt; stir and turn the heat up so that the mixture boils. Once the mixture begins to boil, do not stir for 4 minutes. Once the mixture has boiled for 4 minutes remove from heat and add the soda and vanilla. Stir again. The soda will make the mixture bubble and turn a yummy golden color. Pour the mixture over the bowl of popcorn. Stir the mixture and the popcorn together. A REALLY large bowl helps. Stir until the mixture is attached to the popcorn. My kids like to eat it now, yet it will not stay for a long time unless the popcorn is baked. If you plan to store the popcorn, than you need to bake the popcorn.
You will need two large cookie sheets with sides on them. Put ½ of the mixture on each pan. Bake for an hour on 400 degrees. Stirring every 15 minutes, take out of the oven and let the popcorn cool.