Pour popcorn into 2 glass 9×13 pans. 24 cups of popcorn will fill your pans to the top.
Melt butter in a large saucepan over medium heat.
Add brown sugar, corn syrup, salt and vanilla. Stir and bring to a steady boil.
Boil mixture for 5 minutes, stirring regularly.
Remove mixture from heat and add baking soda. Stir quickly.
Pour caramel over popcorn, mixing well.
Bake at 250 degrees for 1 hour. Stir the caramel corn every 15 minutes to ensure an even coating of caramel as the popcorn bakes.
Cool. Store in a tightly closed container.
Notes
The popcorn does not need to be exactly 24 cups. You can use a little more or a little less. I fill my 2 pans to the top without measuring the popcorn most times.
You can also bake this popcorn in 1 large roaster instead of 2 smaller 9×13 pans.