Combine ½ cup water, sugar and salt in a medium saucepan over medium-high heat. Stir until sugar is dissolved, then remove spoon.
Brush inside edges of pan with water to dissolve any sugar crystals. Reduce heat to medium, then allow mixture to boil, gently swirling pan occasionally. Cook to a light amber color, 20 to 25 min, remove from heat and stir in cream and butter with a clean spoon (mixture will bubble, then settle). Stir in vanilla and set aside to cool. Sauce can be refrigerated for up to a month.