Combine brown sugar, butter, syrup, vanilla and salt in saucepan. Boil 5 minutes, stirring occasionally. Remove from heat and stir in soda and cream of tartar (stir quickly) and pour over popped popcorn in a large mixing bowl. Mix well. Spread onto baking sheets or roasting pan. Bake at 250 degrees for 1 hour, stirring occasionally. Remove from oven and turn onto waxed paper to cool. Store in airtight container.