Preheat oven to 250 degrees (120 degrees C). Coat a large roasting pan with cooking spray. Place the popcorn in the roasting pan and keep warm in the oven.
In a heavy saucepan over medium heat, combine brown sugar, 1 cup butter and 1/2 cup corn syrup. Heat, without stirring, to 250 to 265 degrees F (121 to 129 degrees C), or until a small amount of syrup dropped into cold water forms a rigid ball. Remove from heat and stir in vanilla. Pour syrup over popcorn and stir to coat. Return popcorn to oven.
In the same saucepan, combine chocolate chips with remaining 1/2 cup butter and 2 cups corn syrup. Cook, stirring, over medium heat, until chocolate is melted. Remove from heat and quickly pour over popcorn, stirring to coat.
Return popcorn to oven for 30 to 40 minutes, stirring occasionally. Remove and pour out onto waxed paper lined sheets to cool completely.