Preheat oven to 250°F (120°C).

Line a baking pan with foil.

Place puffed popcorn curls in the prepared pan.

In a large saucepan, combine brown sugar, margarine, and light corn syrup.

Bring the mixture to a boil over medium heat, stirring constantly to prevent burning.

Reduce heat and simmer for 2 minutes, continuing to stir.

Remove the saucepan from the heat.

Stir in baking soda and vanilla extract until well combined.

Pour the syrup mixture evenly over the popcorn in the pan.

Mix the popcorn thoroughly to ensure it is evenly coated with the syrup.

Bake in the preheated oven for 30 minutes.

Stir the popcorn twice during the baking time to ensure even cooking and prevent sticking.

Remove the pan from the oven immediately after baking.

Transfer the coated popcorn from the pan to a clean surface to cool completely.

Break apart any large clumps of popcorn after cooling for serving.