I have learned through my cheese maker friends that ultra pasteurized milk should not be used for this recipe as it is likely not to curdle. As with any cheese, the fresher the milk you start with, the better the result.
Line a large colander with a layer of cheesecloth (or, better yet, use an old, clean t-shirt from which you have cut a big square piece of material) and place it over a large bowl.
Slowly bring milk, cream, and salt to a rolling boil in a heavy pot, stirring occasionally to prevent scorching. Remove from heat, gently stir in the vinegar, and let stand without stirring for 30 minutes. The mixture will have separated into white curds and clear whey.
Pour the mixture into the lined colander, and let it drain for at least 1 hour. After an hour, you will have a tender, spreadable ricotta. After two hours, the mixture will be spreadable but a bit firmer, almost like cream cheese.
Once you achieve the consistency you desire, enjoy immediately or cover and refrigerate; the ricotta will keep for approximately 3 days.