I tried this recipe twice and each time the caramelized sugar ended up like rock candy in the pan when I added the cream, and it wouldn't mix with the cream--it just hardened into a clump. I checked my thermometer w/boiling water to make sure it was reading the temp correctly, and I made sure to tilt the pan so that I could get a good reading since the sugar/water was pretty shallow in the pan. I'm wondering what I did wrong! I used an older metal saucepan and I wonder if the metal is too thin? Do you need a super heavy duty saucepan? Help!