Pop popcorn according to package directions and make sure you get rid of any unpopped kernels. This seems like a lot of popcorn but trust me it will get eaten! Place popcorn in a large bowl...larger than you think you will need.
In a large saucepan melt together the butter, brown sugar, and corn syrup. Bring mixture to a boil. Once it comes to a boil, reduce to medium heat and add the can of sweetened condensed milk. With the rubber spatula, stir constantly for 4-5 minutes scraping the sides and especially the bottom of the pan to prevent any burning. The longer you cook it the firmer it will be. If you like it on the softer side, cook it closer to 4 minutes.
Source: Tiffany Thornton