Cinnamon-Sugar Popcorn

Preheat an oven to 250 degrees F (120 degrees C). Pop the popcorn using an air popper according to manufacturer's directions. Place into a large mixing bowl, and set aside.
Melt the butter in a small saucepan over medium heat. Stir in the sugar, cinnamon, salt, and vanilla, and cook until thick and bubbly. Pour over the popcorn, and stir until the popcorn is evenly coated. Spread the popcorn into a large roasting pan.
Bake in the preheated oven 10 minutes, then turn the heat off, and allow the popcorn to stay in the oven 20 minutes longer. Remove from oven, and cool completely before serving.