From the blog For Love of the Table
2 sticks unsalted butter (225 g)
2 c. packed golden brown sugar (400 g)
1/2 c. light corn syrup (165 g)
1 1/4 t. salt
1 t. baking soda
1 t. vanilla extract
7 quarts popped popcorn
Place the butter in a medium sauce pan along with the brown sugar, corn syrup, and salt. Bring to a full boil (medium to medium-high heat) and boil for 5 minutes—stirring occasionally. Remove from the heat and add the baking soda and vanilla extract. Stir well—the mixture will foam and increase in volume. Immediately pour the mixture over the popcorn (in a large roaster) and stir well until the popcorn is evenly coated. Place in a 200° oven for 1 hour to an hour and 15 minutes (see note), stirring at 15 minutes intervals. Remove from the oven and cool, stirring once or twice as it cools if you don't want big clumps. Store in an airtight container.
Note: The original recipe called for one hour in the oven. Sometimes after an hour the popcorn still seems a bit too sticky to me. When this happens, I give it another fifteen minutes.
http://www.forloveofthetable.com/2013/01/caramel-corn-in-honor-of-national.html