Place the brown sugar, corn syrup, and butter in a saucepan and stir until the margarine is melted. Add the vanilla and stir. Add the sweetened condensed milk and stir all together. Stir and cook slowly over medium heat until it reaches the softball stage. To test this, just get a mug of cold water and drop some caramel in. If it stays somewhat together and not turn the water soupy, it's ready. Pour over a large bowl of air popped popcorn. That's about 1½ c. kernals.
Shape into balls and wrap in plastic wrap, or leave it in the bowl.
Recipe by Mandy's Recipe Box at https://mandysrecipeboxblog.com/caramel-popcorn/