The sauce takes less than 15 minutes to make, but timing is important. Have all your ingredients measured and next to the stove before you start cooking. Be sure to use a larger pot than you think you will need: when you add the cream to the cooked sugar it will foam and bubble—and you want that contained within the pot. When making multiple batches, bear in mind that the sugar will take longer to reach 350 degrees.
Using a thermometer takes the guesswork out, but you can make caramel without it: pay close attention to the color of the sugar as it cooks, and add the cream when the sugar is the deep golden brown color of honey.
Combine the sugar and water in a 5-quart (large) sauce pan. Bring to a boil and continue boiling, stirring occasionally, until the mixture is amber-brown and reaches 350 degrees, about 9 minutes. Remove from heat.
Pour in the cream—the mixture will foam and bubble, so pour away from you. Stir, adding the butter, until both are incorporated and the mixture is smooth. Stir in the salt and allow to cool.