Preheat oven to 300 degrees.
Remove all unpopped kernels from popped popcorn. Put popcorn into a 17x11x2 inch baking or roasting pan.

In a medium saucepan combine sugar, corn syrup, and butter. Cook and stir over medium heat until mixture boils. Remove pan friom heat. Stir in chocolate chips and vanilla. Stir until melted.

Pour hot chocolate mixture over popcorn. Stir gently until coated. Bake for 45 minutes stirring gently every 15 minutes. Spread on a large piece of foil. Cool completely. Store tightly covered for up to one week.