Combine, the butter, brown sugar, and corn syrup in a large saucepan over medium heat. Stir occasionally until butter is melted and mixture is smooth. Increase the heat to medium high and bring to a boil. Once boiling, reduce heat to low and simmer for 10 minutes, stirring occassionally .
Remove from heat and add the sweetened condensed milk, salt, and vanilla extract. Stir until smooth. Pour over the popcorn and stir to evenly coat the popcorn.
You can substitute Pirate's Booty or puffed corn for the popcorn.