In a large saucepan dissolve the brown sugar and salt in the butter over medium heat.
Add the Karo syrup and mix
Pour the sweetened condensed milk in and stir.
Boil the mixture over medium heat until it hits 240 degrees F on a candy thermometer (remember candy temperatures are listed as sea level degree's so adjust according to your altitude - for example I lower all temperatures by 20 degrees F because I live so high up)