Place popped popcorn in a very large buttered bowl and set out buttered wax paper.
Combine sugar, corn syrup, cream of tartar, and butter in a saucepan.
Cook over medium heat, stirring, until the syrup reaches 270°F.
Remove from heat and stir in baking soda carefully.
Pour hot syrup over popcorn and toss to coat.
Fold in candy corn while the mixture is still warm.
With buttered hands, shape mixture into balls and place on wax paper to cool.
Notes
Butter your hands before shaping so the syrup does not stick. If the mixture stiffens before shaping, warm the bowl briefly in a 200°F oven. Do not refrigerate; humidity makes the coating sticky. Wrap popcorn balls individually and store at room temperature for 3 to 4 days.