In a large saucepan, melt 1½ cups butter. Add 2⅔ cups brown sugar and 1 cup golden syrup, stirring to dissolve brown sugar. Bring to a full rolling boil. Boil and stir 1 minute. Remove from heat and quickly stir in 1 teaspoon vanilla. Pour caramel mixture over 24 cups popped popcorn, stirring lightly. Cool on waxed paper then break it up.