In a heavy saucepan, combine the sugar and the water. Stir to combine and then cook over medium heat until the sugar dissolves; about 4-5 minutes. There is no need to stir, simply swirl the pan over the burner occasionally as the sugar dissolves.
Raise the heat to medium high and cook the sauce, swirling the pan occasionally, until it is amber colored; about 7-8 minutes. Add the butter, be prepared for it to bubble and sputter, lightly stir or swirl to melt, do not scrape the sides of the pan or splash sauce up the sides of the pan. (This will cause crystallization, if you scrape the uncooked sugars back into the sauce.)
Once the butter is melted, remove from the heat and add the vanilla, heavy cream and salt. It will bubble again, but will calm quickly. Stir to combine (again, not scraping sides of the pan) and then let cool before transferring to a jar and refrigerating for storage. Enjoy!