1 cup popcorn kernels, unpopped
1 cup (2 sticks) unsalted butter
½ cup light corn syrup
2 cups packed light brown sugar
1 tsp. kosher salt
½ tsp. baking soda
1 tsp. vanilla extract
Pop popcorn using your preferred method. (I swear by my Whirley Pop!) Place the popped popcorn in a very large bowl (or two large bowls). Make sure there is enough room in your bowls for tossing the popcorn.
Preheat the oven to 250° F. Meanwhile, to make the caramel, melt the butter in a 2-quart saucepan set over medium heat. Once the butter is melted, mix in the corn syrup, brown sugar and salt. Increase the heat to medium-high and bring the mixture to a boil, stirring frequently.
Once the mixture reaches a boil, let the mixture continue to boil for 5 more minutes without stirring. Remove the mixture from the heat, stir in the baking soda and vanilla extract. The mixture will bubble up and become frothy. Pour the caramel sauce over the popcorn in the bowl(s) and toss to coat thoroughly. (I found my salad spoons very useful for this step.)
Spread the popcorn evenly onto two large baking sheets lined with parchment paper. (It is okay if there are some clumps of caramel throughout. It will melt and become better spread out while baking.) Bake for 40-50 minutes, removing from the oven to toss/stir every 10 minutes. After 40 minutes, test a cooled piece of popcorn. If it is not completely crispy (i.e. still slightly mushy when you bite it), bake for 10 minutes more.
Remove the baking sheets to a wire rack and allow popcorn to cool completely before breaking apart for serving or packaging.