1. Pop the kernels in a pot or a hot air popper; one cup kernels should yield about 10 cups popped corn. Salt lightly and let cool. Place the popcorn in a large bowl that has been sprayed with nonstick cooking spray.
2. Combine the sugar, corn syrup, water, salt, and butter in a medium saucepan over a medium-high flame. Stir occasionally until the sugar dissolves and the butter melts.
3. Insert a candy thermometer and boil the candy, without stirring, until the thermometer reads 244°F (118°C). Once the candy reaches the proper temperature, remove the pan from the heat and stir in the vanilla extract.
4. Pour half of the hot sugary syrup over the popcorn in the bowl and stir until it’s well distributed. (This is when you should stir in any other add-ins — nuts, candy, etc.)
5. Once the mixture is mixed well, pour in the rest of the sugar syrup and stir until everything is coated.
6. Wet your hands or spray them with nonstick cooking spray, then form the popcorn mixture into large balls about 3 inches in diameter. Re-wet or re-spray your hands as needed. Don’t be afraid to firmly press the balls together as you’re forming them so that they don’t fall apart.
7. Let the balls cool to room temperature, then serve.
These popcorn balls taste best on the day that they are made, but can be enjoyed up to 24 hours. (After this period, they begin to taste stale.) If you’re not eating them immediately, wrap them individually in cling wrap.