Melt butter. Stir in sugar, salt and syrup. Stir while coming to boil. Boil 5 minutes, without stirring. Remove from heat. Add soda, vanilla and stir. Put popcorn in large pan and pour mixture on. Mix well. Pile on 2 large cookie sheets. Bake 1 hour at 250 degrees. Stir every 15 minutes. Cool. Break apart and store in air tight container.
1 cup unsalted peanuts may be added.