There have been discussions in the past about the dirth of real hot fudge sauce in SF. I was in Isabella's, at Castro/24th in Noe Valley, and noticed that they advertise a house-made sauce made of Scharffenberger chocolate.
They feature Double Rainbow ice cream, for the record.
I was craving a milkshake and enjoyed the one they made, although it was gi-normous. The gelato place at Sanchez wanted $6.75 for theirs, which seemed a bit much for a milkshake, I was fine with the traditional ice cream shake I received for ~$4.