Pop plain popcorn using an air popper or stovetop. Transfer popcorn to a clean bowl in batches (if needed) to get rid of any unpopped kernels.
Cook the brown sugar, Karo syrup, and ½ cup of butter over the stove on medium high heat and bring to a boil.
Stir in the sweetened condensed milk. Stir continually, around 3-4 minutes, until soft-ball stage (so it passes the cold-water test, or about 235℉ on a candy thermometer).
When it is finished cooking, take your pan off the stove and pour the caramel sauce over your air popped popcorn. Mix until all the popcorn is completely coated. You will need a very large bowl to mix it all together.
Notes
Update popcorn balls note at the bottom: To make popcorn balls, let the caramel popcorn cool for about 5-10 minutes until warm but still pliable. Lightly butter or spray your hands with nonstick cooking spray, then press into balls and wrap individually. These will still be gooey and chewy in texture.