Add water to a 2-qt saucepan. Gently add the sugar to the center of the pot - it will mound, that's fine. Cover the pot and bring to a boil over high heat.

Once boiling, uncover the pot and insert a candy thermometer. Continue cooking until the mixture registers 300 F and is just starting to develop some color, about 15 minutes.

Reduce heat under the pot to medium and cook until the syrup is amber and registers 350 F on the thermometer, about another 5 minutes.

Meanwhile, pour the cream into a small saucepan and bring to a simmer. If it simmers before the syrup is ready, just take it off the heat and set aside.

Remove the caramel from the heat and add about 1/4 of the warm cream to the pot. It will bubble furiously so be careful. Once the bubbling subsides, add the remaining cream. When it stops bubbling, whisk gently to incorporate fully. Add the butter and the salt and whisk to combine.

Refrigerate up to 1 month.