1In a heavy-bottomed nonreactive pot, combine the milk, buttermilk and salt over medium-low heat. After about 20 minutes, you will start to see steam rise from the milk; at that point give it a gentle stir with a metal spoon.
2After about 10 more minutes you’ll begin to see curds—the clumpy white mass—rise to the surface. (Use an instant-read thermometer to monitor the milk’s temperature, which should be between 170 and 180 degrees.) Once you see curds floating, cook for 5 more minutes. At that point the curds will begin to sink, and that means it is time to strain the mixture.
3Line a colander with a large piece of cheesecloth that has been folded over a couple of times. Set the colander in the sink. Pour the curds into the cheesecloth, leaving as much of the whey—the liquid—in the pot as possible. Gather the edges of the cloth and tie the bundle to the faucet; let the curds drip for 5 minutes.
4Transfer the ricotta to a food processor and add the lemon zest, cream and more salt if desired. Pulse until smooth and combined. Ricotta can be stored in an airtight container and refrigerated for up to two days.