Preheat the oven to 200ºF. Grease two baking sheets with cooking spray.
Cook ⅔ cup of popcorn kernels or buy and measure out 6 quarts into a large mixing bowl.
Melt the butter in a medium saucepan set over medium heat. Stir in the brown sugar, corn syrup, cream of tartar and salt. Bring the mixture to a boil, stirring constantly, then allow the mixture to boil for about 5 minutes without needing to stir it. You want the caramel to get to a hard-ball stage which is 250 degrees if using a candy thermometer. If not, just make sure you boil for 5 minutes.
Remove from heat and add baking soda and stir.
Immediately pour the caramel over the popcorn, stirring to coat the popcorn.
Pour the coated popcorn onto the 2 baking sheets, spreading it out into an even layer across.
Bake the caramel corn for one hour, stirring every 15 minutes, and changing which pan is on top if you remember.
Remove the caramel corn from the oven and sprinkle with flaked sea salt. Then stir it one more time before allowing it to cool completely. Break it into pieces and serve.