Position a rack near the center of the oven and preheat the oven to 300 degrees F. Line a rimmed baking sheet with a piece of foil (grease with cooking spray) or a silicone mat. Lay the prepared popcorn on the sheet in a single layer, depending on the size of your baking sheet, you may need two.

Melt the butter in a medium saucepan over medium-high heat. Add the brown sugar and corn syrup and stir to combine. Bring to a rolling boil, without disturbing further, reduce the heat to low and let simmer for 5 minutes. Stir, them continue to cook for an additional 3-4 minutes, making sure to stir every 30 seconds, until the mixture deepens in color to a golden brown. Remove from heat and whisk in the salt, vanilla, and baking soda - be careful as the mixture will become violent when vanilla and baking soda is added.

Drizzle the prepared caramel over the popcorn and using a nonstick spatula combine everything until most of the popcorn are coating with caramel. Spread the mixture evenly on the baking sheet and bake for 14-21 minutes, stirring every 7 minutes, until the caramel corn turns a deeper amber color. Remove from oven and transfer to wax paper lined baking sheets (countertop is fine too). Once the caramel corn comes to room temperature, break up any large clusters of popcorn. Store in an airtight container for up to one week.