This weekend I tried making salted caramel following a recipe from Bon Appetit's Test Kitchen (The caramel part from https://www.bonappetit.com/recipe/salted-caramel-chocolate-tart).
I noticed a little foam during the caramel formation but it did not give me much trouble at the moment, but later, when I added the butter, everything has gone awfully bad.
I took it off the stove to add the butter but it started to gain a sandy texture and a light yellow color. I put it on the stove again and kept adding butter hoping to solve it but it did not help.
In the end, I resolved by using half a glass of water, that lowered the temperature and melted everything back to a smooth deep amber sauce. After adding the cream and trying to reduce it to remove the added water, it went ok.
What did I do wrong? What happened?
P.S. In the recipe it asks for cream of tartar but I did not have it so I did not use it, is it the cause? I've seen other recipes without it, so I thought it was ok.