I'm taking some in for my class party tomorrow and we needed some for us...So, I'm on my second batch. It's as good as I remember.
Mix 1 cup butter, 2 cups brown sugar, 1/2 cup dark corn syrup, and 1/2 teaspoon salt in a saucepan. Bring to a boil on high, stirring constantly. Boil 5 minutes without stirring. Remove from heat and add 1/2 teaspoon baking soda and 1 teaspoon vanilla. Pour over 5-6 quarts popped popcorn in baking pan and stir. Bake at 250 for 45 minutes, stirring every 15 minutes. Pour out on waxed paper and break apart as it cools.