Put the sugar, corn syrup, and water in a medium saucepan.
Cook over medium-high heat, brushing down the sides of the pan as needed with water, until the sugar dissolves.
Continue to cook, swirling the pan but not stirring, until the sugar is amber in color, about 8 minutes.
Pull pan from the heat and carefully add the cream and butter. Take care; it may spatter. Whisk until smooth.(You may find that some of the sugar mixture will stick to the bottom of your whisk- don't worry the sauce will still be wonderful!).
Stir the chocolate, vanilla, and salt into the hot caramel until smooth. Serve sauce warm.
Store in a sealed container in the refrigerator for up to 1 week. Reheat slowly, in a microwave at 30 second intervals, stirring, until warm and saucy. Be careful not to over heat or butter can separate from the caramel sauce.