Using 2 jelly roll pans (or cookie sheets), spray with non-stick vegetable spray. Divide the popped popcorn evenly between the two pans.
In a medium saucepan, heat brown sugar, butter, corn syrup, salt, and vanilla extract over medium heat stirring continually. It should be bubbly around the edges of the mixture. After approximately 5 minutes, remove the mixture from the heat and quickly stir in the baking soda.
Pour divided amount of mixture evenly over both pans of popcorn. Stir coating the popcorn.
Place pans in oven and bake for 1 hour stirring the popcorn well every 15 minutes. Let cool and store in airtight container(s).