Preheat oven to 275°F.

Spread popcorn in TWO large roasting pans (I use the disposable aluminum turkey roasting pans).

In a medium saucepan, combine the butter, brown sugar, and Karo syrup. Cook over medium heat, stirring only until the butter’s fully incorporated (then you don’t need to stir anymore). Cook until mixture reaches 255°F (essential to use a cooking/candy thermometer).

Remove from heat and stir in vanilla. Then dump in the baking soda and quickly stir until the mixture foams up and it all becomes a lighter brown color, pretty much the same color as peanut butter (about 30 seconds of stirring).

Quickly pour caramel evenly over all the popcorn. Toss and mix until all the popcorn is fairly evenly coated. (Won’t be super-well coated at this point.)

Place in oven for 10 minutes, then toss/stir again to better coat the popcorn. Bake another 10 minutes.

Remove from oven, stir again, and quickly sprinkle with kosher salt. Toss for about a minute, allowing the caramel corn to cool off, scraping the bottom of the pan to keep it from sticking.

Cool completely and store in airtight containers.

POPCORN BRANDS: I grew up in Indiana, where all the best popcorn is grown, and I tried dozens of brands over the decades. My favorite is the brand sold by the Boy Scouts. Large, consistent pop, great flavor, and perfectly shaped for candy coating. When I run out of Boy Scouts popcorn and can't get more quickly, I buy Yoder brand at the grocery because it's nearly identical. Those are my preferences, but feel free to use whatever popcorn you prefer, as long as it's not microwave!