Pop your popcorn in an air popper machine. My machine only allows 1/2 cup of kernels at a time so I had two separate batches that I had to pop.
Cook the brown sugar, Karo syrup, and 1/2 cup of butter over the stove on medium high heat and bring to a boil.
Add the sweetened condensed milk. Stir continually for about 3 to 4 minutes until it gets to the soft ball stage (it clings together when you drop some in cold water). It will burn if you cook it too long, so make sure you are stirring continuously and watch it closely.
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
You could easily roll this popcorn into popcorn balls when it is cool enough to handle. Then wrap them individually in plastic wrap.