In a medium-size stainless steel saucepan, stir the sugar and cream of tartar together with 2 tablespoons (30ml) of water. Turn the heat on to medium and cook, without stirring, until the mixture melts. Swirl the pan gently to move the sugar around to prevent burning as it cooks. When the sugar is melted and has turned deep golden, turn off the heat and whisk in the butter. The butter will bubble. Keep whisking until melted, smooth and well combined. Gradually whisk in the heavy cream until smooth. Pass the caramel through a fine mesh sieve into a bowl. Stir in the vanilla and salt. The sauce will thicken as it cools. Cover and store in the refrigerator for up to a week.