In a medium saucepan over medium-low heat add the light brown sugar and the water with cream of tartar. Mix with a spoon until the sugar begins to boil. Cook undisturbed for 8-10 minutes until the sugar turns deep amber and wisps of smoke rise from the surface.
Meanwhile wash over the sides of the pan with a brush to avoid sugar crystals from sticking. Remove caramel from the stove and stir in the butter. Gradually add the cream, vanilla and salt. Return to heat and allow to cook until the mix boils again, 4-5 minutes.
Sieve the caramel through a fine mesh sieve into your preferred container. This will separate any undissolved and unnoticed sugar crystals from the sauce. Allow to cool completely and enjoy.