Melt butter in a saucepan over medium heat. Stir in the brown sugar and baking soda. Bring to a boil, stirring constantly. Boil for 5 minutes, stirring occasionally.
Remove from heat and stir in vanilla. Pour over popcorn and stir until evenly coated.
Spread the popcorn onto a greased baking sheet and bake for 45 minutes, stirring every 15 minutes.