Preheat oven to 200F. Boil together butter, brown sugar and corn syrup. Add baking soda and boil for 5 full minutes. Be careful to stir this constantly, it will want to boil over the sides of the pan. Remove from heat after 5 minutes and add vanilla extract and salt. Line a pan with foil and bake for 1 hour, stirring every 15 minutes. When done, pour out onto another piece of foil or cooking parchment and allow to cool When cool, break into pieces.