Combine brown sugar, butter, cream of tartar, salt and Karo syrup in a large saucepan. Cook and stir to hard-ball stage. Remove from heat and stir soda and vanilla. Mixture bubbles up quite a bit. Pour over popped corn. I use a large metal bowl. Stir till popcorn is covered.Bake in 200F oven for 45 minutes. Stir every 10-12 minutes.Remove from oven and spread onto wax paper to cool.ENJOY!