In a large pot over medium heat, melt the butter. Add the popcorn kernels and cover the pot with a lid. Cook until all of the kernels have popped, shaking occasionally to prevent burning.
In a small saucepan over medium heat, combine the brown sugar, corn syrup and baking soda. Cook until the mixture boils and thickens slightly, stirring frequently.
Remove from heat and stir in vanilla extract.
Pour caramel mixture over popcorn and mix until evenly coated. Spread onto a greased baking sheet.
Bake at 250°F for 30 minutes, stirring every 10 minutes.