Pop popcorn according to directions. You are going to want to be sure to get ALL of the unpopped kernels out. I use a 2-bowl method. You’ll need 2 large bowls. Dump the popcorn into bowl #1. Give it a little shake and let the unpopped kernels fall to the bottom. Using your hands transfer the popcorn to bowl #2. This is best done in small batches. I usually complete one bag while the next bag is popping in the microwave.
In a large saucepan combine butter, brown sugar, and light corn syrup until melted and comes to a boil.
Reduce to low/medium heat and add the sweetened condensed milk. Stir continuously for about 5 minutes. The longer you stir the firmer your caramel will be.
Pour caramel over the popcorn and stir until incorporated. You will feel like it is not working at first but keep at it and the caramel will coat the popcorn.
Some great add-ins would be nuts, M&Ms, pretzel pieces, or chocolate chips.