Line a colander with 4 layers of cheesecloth, and secure it with 3 or 4 clothespins. Set the colander inside a bowl.
In a large saucepan over medium-high heat, with a thermometer handy, combine the whole milk and buttermilk and heat, stirring nearly constantly, until the temperature reaches 180 degrees F. When you reach 170 to 175 degrees F, you'll start to see fine, little curds separating from the whey. At 180 degrees F, turn off the heat, and skim the curds from the whey using a finely slotted spoon, dropping the curds into the cheesecloth-lined colander. (Save the whey for another use, such as breadmaking.) Gather the cheesecloth around the curds and tie it with a 2-foot length of string. Gently squeeze to remove more, but not all, liquid from the cheese, and then hang over the sink or a bowl for 20 to 30 minutes to drain it a bit more. (I use the string to tie the bag to my faucet.)
Remove the ricotta from the cheesecloth, spoon it into a container, and stir in the salt. Serve ASAP, preferably without refrigerating.
For this recipe, where the flavor of the milk is so important, use the highest quality you can find. Your best bet is a local dairy that is likely to pasteurize its milk more gently than a factory brand, and steer clear of homogenization. For me in New York, that's Ronnybrook, which is available at many farmers' markets and better supermarkets, and hails from just a couple of hours north of the city.