Heat oven to 250 degrees Fahrenheit.
Pop 1 cup of popcorn and put it in a buttered roast pan.
Melt 1 cup of butter. Stir in 2 cups brown sugar,1/2 cup corn syrup and 1 tsp salt.
Bring to a full boil stirring constantly. Let mixture boil for 5 minutes and remove from heat. Do Not Stir.
Stir in 1/2 tsp soda and 1 tsp vanilla. Pour over popcorn and mix together well.
Bake for 1 hour stirring every 15 minutes. Spread out onto parchment paper to cool.