Preheat oven to 200°F.

Make popcorn using an air popper, or a favorite popping method (stovetop/microwave, etc.). Once popped, remove/discard any uncooked kernels. Put popcorn in a LARGE bowl; set aside. NOTE: It's important to have popcorn ready before cooking caramel sauce!

Melt butter in a large saucepan. Add brown sugar, corn syrup and salt. Stir to combine. Bring sauce to a boil on medium heat. Once boiling, continue boiling for 5 minutes, stirring often. Sauce will get "foamy" as it cooks, and will rise in the pan (which is why you use a large saucepan). Keep on stirring! Near the 5 minute mark, it will darken in color, as well. Once it's boiled 5 minutes, remove pan from heat. Stir in baking soda and a pinch of cream of tartar (this helps create crunchy texture-don't skip this).

Immediately pour hot caramel over popcorn. Use a large spoon and stir, until all popcorn is well coated in caramel sauce. Just keep stirring! TIP: Sauce is VERY HOT, so be careful.

Spread coated popcorn onto 2 large cookie sheets. Try to keep popcorn in a single layer, separating pieces with your fingers to break up large "globs". Bake at 200°F for 90 minutes (1½ hours), stirring every 20 minutes, to keep pieces moved around and broken up (for even heat distribution). Once done, remove from oven; let caramel corn cool completely. Break up any large pieces before serving. Store in airtight container to keep it crunchy for weeks! Enjoy.