Do NOT use ULTRA-pasteurized heavy cream or heavy cream with gums, stabilizers, or thickeners (such as gellan gum or carrageenan) in the ingredient list. Use only pasteurized or non-pasteurized heavy cream or heavy whipping cream, with at least 35% butterfat, and no additives. 

To make the clotted cream, you will need a probe thermometer, plus a double boiler or a medium to large metal or glass mixing bowl set on top of a medium saucepan. Just be sure to use a wide enough bowl that your heavy cream has a lot of surface area while heating.

DO NOT stir the cream at all, ever, even once during this process. A few gentle sloshes here and there, while you move the container, are OK, but stirring or swirling to incorporate the cream is not.

You should expect to get a little less than half the volume of clotted cream than the volume of heavy cream you started with. So if you start with 16 ounces of heavy cream, expect about 6 ounces of clotted cream.

I have found that occasionally my clotted cream turns out kind of dry and waxy, even with good quality heavy cream – still tasty, but not the best texture. I believe it either has to do with the brand of cream or possibly seasonal variations in cream production. If you run into the same issue, simply stir a spoonful or two of the leftover thin liquid into the clotted cream. After a few hours, it should incorporate nicely, smoothing and loosening the texture of the clotted cream.