Place the sugar, salt, cream of tartar (if you're using it) and water in a deep, heavy-bottom saucepan; it should be at least 1 1/2-quart capacity, as the syrup will bubble up during preparation. Stir to combine, and heat over medium-high heat, swirling the pan to help it along, until the mixture starts to bubble.
When the mixture comes to a boil, reduce the heat to medium and watch carefully. The syrup will begin to turn golden around the edges. Shake the pan in a swirling motion to keep the syrup moving, and to prevent the edges from burning before the center colors. Stir the edges in towards the center if necessary.
After a couple of minutes the syrup will begin to darken. If it looks like it's getting dry/crystallized on top, stir until it's syrupy again. As soon as it's a rich light-amber color, remove it from the heat. Gradually drizzle in the butter, stirring all the while, until fully combined. The whole process to this point will probably take no more than 7 to 10 minutes, so don't walk away.
Add the cream a couple of tablespoons at a time, stirring until fully incorporated after each addition. Stir until smooth. Add the vanilla or flavor of your choice; butter-rum is tasty. Or stir in a tablespoon of rum.
When the sauce is smooth, let it sit in the pan for a couple of minutes before pouring it into a glass or ceramic container to cool. It'll be very thin at this point but will thicken as it cools, and thicken further once chilled. Cool to room temperature, and store in the refrigerator. Reheat briefly before serving.
Sauce will stay good in the refrigerator for at least several weeks.
Tips from our Bakers
- Why add cream of tartar to the sugar syrup? It's extra insurance against the syrup crystallizing. Many readers successfully make this sauce without it; but if you ever find yourself dealing with crystallized (instead of smooth) syrup, be sure to add the cream of tartar next time around.
- Don't let the sugar syrup become too dark before removing it from the heat; it'll continue to cook a bit even after it's off the heat. The darker the syrup the richer its flavor; but bitter/smoky notes start to creep in if you let it darken beyond medium amber.
- For a twist, try Salted Rosemary Caramel Sauce. Place 3/4 cup sugar, 2 tablespoons boiled cider, 1/2 cup heavy cream, 2 tablespoons unsalted butter, 1/4 teaspoon salt, and one 4" sprig fresh rosemary in a medium saucepan. Bring to a boil, stirring once or twice at the beginning until the sugar dissolves. Once dissolved, cook over medium heat without stirring until the mixture reaches 230°F. Remove from the heat and strain into a heatproof container.