1 cube (1/2 cup) butter
1 c. sugar
3/4 c. light corn syrup
1 Tbsp vanilla (yes this is 1 Tbsp and not a tsp.)
Melt butter in a medium saucepan. Add sugar and corn syrup, and bring to a boil over medium heat. While stirring constantly, boil for 1 minute. Remove from heat and add vanilla. Pour over air-popped popcorn and stir well until all pieces are coated.