I start with 16 cups of popped popcorn and put it in my turkey roasting pan.
Once the popcorn is ready, I mix up the caramel syrup in a 3 qt. saucepan. Combine 1-1/2 cups brown sugar, 12 tablespoons butter (1-1/2 sticks), 6 tablespoons light corn syrup, and 1/2 teaspoon salt. Cook and stir over a medium heat until the butter melts and mixture comes to a boil. Then let it cook without stirring (boiling lightly) for 5 minutes more. Turn off the heat and stir in 1/2 teaspoon baking soda and 1/2 teaspoon vanilla.
Pour syrup over popcorn in pan and gently stir to coat popcorn as evenly as you can.
Then put in a 300° oven for 15 minutes. Take it out and stir. Return to oven and bake for 8 minutes more. Remove from oven and cool.
Be sure to stir the caramel corn frequently as it cools or you will end up with one heckuva big caramel corn blob that you'll have to break apart with a small axe. Makes 16 cups and keeps (if you hide it) forever in an air-tight container.