Makes about 20 cups.
Preheat the oven to 250 degrees. Pop the popcorn, discarding any unpopped kernels.
Arrange the popcorn evenly in a large roasting pan. Place in the oven.
Combine the butter, brown sugar, corn syrup, vanilla paste and salt in a large saucepan. Bring to a boil slowly. Boil for 5 minutes over medium heat, stirring constantly; do no burn. Stir in the baking soda and cream of tartar, which will cause the sugar mixture to rise. Pour over the warm popcorn, stirring until coated. Bake for 1 hour, stirring every 15 minutes.