Preheat oven to 250 degrees F. Place popped corn in large open roasting pan; set aside.
In 2 quart saucepan over medium heat, heat sugar, corn syrup and butter to boiling, stirring constantly. Remove saucepan from heat; stir in chocolate pieces and vanilla extract until chocolate is melted. Pour hot chocolate mixture over popped corn, stirring to coat well.
Bake popped corn 1 hour, stirring occasionally. Spoon chocolate covered popcorn into another large non-stick roasting pan or onto waxed paper to cool, stirring occasionally to separate.