Preheat the oven to 300 degrees. Line a large baking sheet with parchment paper or a silicone mat. Melt the butter, sugar, syrup, and 1 teaspoon salt in a medium size saucepan over medium heat. Stir to combine and bring to a boil. Allow it to boil for 4 minutes without stirring. While the caramel is simmering, pop the popcorn in an air popper. (Don't have an air popper? Plain unflavored microwave popcorn will work too, you'll probably need two bags worth for about 16 cups worth.) Remove any unpopped kernels from the bowl.
Pour the caramel sauce over the popcorn. Stir to coat thoroughly. Pour the popcorn onto the lined baking sheet. Sprinkle with the remaining salt and stir gently. Bake for 30 minutes, stirring every ten minutes. Remove from the oven and spread across a parchment lined counter. Cool completely before storing in an airtight container. Enjoy!