When making homemade caramel sauce, there are two important rules: First, have patience. It will take a while for the sugar to caramelize and achieve the deep amber color you’re looking for, but keep an eye on the pot. It can go from your desired color to burnt very quickly. This ties into the second rule of caramel making: Have all of your ingredients measured out and within arm’s reach. Once your sugar is caramelized, immediately remove the pan from the heat and slowly add the cream. It will bubble up rapidly, but that means you’re doing it right.
In a medium saucepan with high sides, mix the sugar with ¼ cup of water. Bring to a simmer over moderate heat. Swirl the pan, without stirring, until the sugar turns a deep amber color, about 10 minutes. Watch the sugar carefully so that it doesn’t burn. If crystals start to burn on the sides of the pan, use a wet pastry brush to wash down the sides.
Remove the pan from the heat and slowly add the heavy cream. Be careful as the cream will bubble up quickly.
Off the heat, add the whiskey, salt and butter. Stir with a wooden spoon until the butter is melted, about 1 minute.
Pour the caramel sauce into a heatproof 1-pint jar and let cool completely. Refrigerate until ready to use. For serving, remove the lid and microwave on high for 1 minute.
MAKE AHEAD: The caramel sauce can be made up to 2 weeks in advance.