Bring the brown sugar and corn syrup to a boil. Stir constantly and add the butter pieces and sweetened condensed milk. Continue stirring constantly over medium heat. Cook to a semi-firm ball. If I drop some caramel in a cup of cold water and if it can be made into a semi-firm ball...it's ready. This makes a lot of caramel for popcorn. I like to buy "Nu-crisp" already popped popcorn at Harmon's grocery store, or a similar grocery store. The package I used today is called "Corn Pops" (not the cereal corn pops) and a package of buttered popped popcorn. Put the popped popcorn and/or corn pops in a very large greased/buttered bowl so you are ready to stir the caramel over 1 pound of popcorn very quickly. You can add peanuts, pecans or almonds, enjoy.