In a large heavy saucepan, combine the white sugar, corn syrup, and salt. Bring to a boil and heat to between 234 and 240 degrees F (112 to 116 degrees C), or until a small amount of syrup dropped into cold water forms a soft ball that flattens when removed from the water and placed on a flat surface.
Stir in the butter and heavy cream carefully - the mixture will bubble up. Remove from the heat and stir in the vanilla. Cool slightly before dipping apples or other fruit.