Make The Caramel: Stir together the sugar and 1/4 cup water in a medium saucepan until the sugar is completely moistened. Brush the insides of the saucepan with a wet pastry brush to remove any sugar granules then place the pan over medium-high heat and bring to a boil.Without stirring but swirling the pan occasionally, let the sugar boil until it turns amber in color, about 5 minutes. Give the pan a few good swirls to make sure it’s evenly cooking.
Add The Cream: Remove from heat. Immediately add the cream and butter and swirl until the caramel is smooth and uniform in color. Return to heat and bring to boil, constantly whisking until smooth. Whisk in vanilla and salt, remove from heat, and set aside until ready to use.