1. In a large saucepan on medium heat, combine sugar, Karo syrup, salt and condensed milk. Stir constantly. Using a candy thermometer, bring to a soft boil which is 240 degrees.
2. Once at a soft boil, add butter. Continue to stir constantly. Bring back up to a soft boil.
3. Remove from the heat and add vanilla. Store in the fridge and heat up on stove or microwave when ready to eat.