Begin by preheating the oven to 250 F/120 C.
Make the popcorn either with an airpopper or the old-fashioned way on the stovetop.
In a pot combine the brown sugar, corn syrup, butter, and salt. Set the burner to medium heat. Stir occasionally and bring the mixture to a boil. Once it boils, allow it to boil for 5 minutes without stirring.
After the 5 minutes remove it from the heat and add the baking soda and vanilla extract and stir them in. The caramel will bubble.
Pour the caramel over the popcorn half at a time then stir the popcorn and then the other half. Stir until the popcorn is evenly coated with the caramel.
Spread the caramel popcorn out onto a silicone mat-lined sheet pan. Or you can use parchment paper. Bake for 15 minutes, then stir the mixture, then bake for 15 minutes, then stir, etc. Do until the total time is 60 minutes. Allow the caramel popcorn to cool.
Break it apart and serve! Store in an airtight container at room temperature. It should last a few months. Enjoy!