Melt 1 cup butter, 1 cup brown sugar, ½ cup white sugar, ½ cup corn syrup, and ½ tsp salt, boiling for 5 minutes.
Remove from heat, stir in ½ tsp baking soda and vanilla.
Pour over 8-12 cups of popcorn.
Bake at 250°F (120°C) for 15-60 minutes for a crunchier, non-sticky texture.