In a heavy-bottomed saucepan, add the sugar in a large mound, then add the water. Do not mix.
Heat the sugar on medium until it begins to dissolve in the water and continue to cook undisturbed until the sugar begins to turn to a deep amber color. As the sugar cooks, use a wet pastry brush to clean the sides of the saucepan.
Add the butter and heavy cream as soon as the caramel reaches that amber color. Be careful as the mixture will bubble a lot. Stir until well combined.
Continue to cook for an additional 5 - 10 minutes for the sauce to thicken.
Stir in the vanilla extract and salt.
Allow to cool to room temperature before using.
Notes
Store in an airtight jar for up to 2 weeks. To thin the sauce, add more cream. To thicken, cook the sauce longer. Be sure to check the sauce thickness when it is room temperature as the sauce thickens as it cools.