Caramel Popcorn

3 qts popcorn popped
3 C unsalted mixed nuts (optional)
1 C packed brown sugar
1/2 C Karo light or dark corn syrup
1/2 C margarine or butter
1/2 tsp salt
1/2 tsp vanilla extract
1/2 tsp baking soda

In a large shallow roasting pan combine popcorn and nuts. Place in a 250-degree oven while preparing glaze. In a heavy two-quart saucepan, add sugar, corn syrup, margarine, and salt. Stirring constantly, bring to boil over medium heat. Without stirring, boil for five minutes. Remove from heat and stir in vanilla and baking soda. Pour caramel over popcorn mixture, stirring to coat. Bake at 250 degrees for one hour, stirring occasionally. Cool, break apart, and store tightly covered.