Pop corn and remove unpopped kernels, spread on a baking sheet, and keep warm.
Butter the hands and the wooden spoon before starting.
Combine corn syrup, sugar, and butter in a medium saucepan over medium heat.
Stir until the mixture bubbles, then stop stirring and let it boil for 2 minutes.
Remove from heat, quickly add vanilla, salt, and baking soda (mixture will foam).
Pour over warm popcorn immediately and mix with a wooden spoon.
Form into baseball-sized balls with buttered hands.
Place on parchment paper and cool completely (30 minutes).