Select a sieve or colander with a wide surface area so the curds will cool quickly. Rinse a large piece of cheesecloth or muslin with cold water, then fold it so that it is 6 or more layers, and arrange it in the sieve or colander placed in the sink.

Pour the milk and buttermilk into a large nonreactive saucepan. Place over high heat and heat, stirring the mixture frequently with a rubber spatula and making sure to cover the whole pan bottom to prevent scorching. Once the mixture is warm, stop stirring. As the milk heats, curds will begin to rise and clump on the surface. As the curds begin to form, gently scrape the bottom of the pan with the spatula to release any stuck curds.

When the mixture reaches 175° to 180ºF, the curds and whey will separate. The whey looks like cloudy water underneath a mass of thick white curds on the surface. Immediately remove the pan from the heat. Working from the side of the pot, gently ladle the whey into the prepared sieve. Go slowly so as not to break up the curds. Finally, ladle the curds into the sieve. Lift the sides of the cloth to help the liquid drain. Resist the temptation to press on the curds. When the draining slows, gather the edges of the cloth, tie them into a bag, and hang the bag from the faucet. Continue to drain until the dripping stops, about 15 minutes.

Untie the bag and pack the ricotta into airtight containers. Refrigerate and use within 1 week.