9 cups plain popped popcorn
1 cup granulated sugar
1/3 cup light corn syrup
1/3 cup water
1 teaspoon distilled white vinegar
1 1/2 teaspoons kosher salt
4 tablespoons unsalted butter (1/2 stick), cut into small pieces
3/4 teaspoon ground nutmeg
3/4 teaspoon vanilla extract
1/2 cup toasted pecans, coarsely chopped
Coat a large heatproof bowl with butter, vegetable oil, or cooking spray and place popcorn in the bowl. Place sugar, corn syrup, water, vinegar, and salt in a medium saucepan and stir to combine. Place over high heat, stirring until sugar is dissolved, about 2 minutes. Bring to a boil and cook until mixture registers 260°F on a candy/fat thermometer, about 5 to 7 minutes. Remove from heat and stir in butter, nutmeg, and vanilla until melted and smooth. Immediately drizzle sugar mixture over popcorn. Add pecans and stir continuously with a rubber spatula, scraping the bottom of the bowl, until popcorn is thoroughly coated and cool enough to handle, about 3 minutes. Using buttered or oiled hands, tightly press mixture (it may still be a bit warm) into 3-inch rounds. Place on waxed or parchment paper to cool completely, about 15 to 20 minutes.
The popcorn balls will last up to 5 days when stored at room temperature in an airtight container.