Step 1: Heat milk and whey. Set a 20-qt. canner on the stovetop and insert canning rack upside down (handles down). Fill canner with water to 1 in. below top ring. Bring to a boil, covered. Fill a sink with cold water (this will be your cool-down spot should you need it). Set an empty 8-qt. pot on upturned rack in canner (it will float a little). Pour whey if using, milk, and salt into pot and insert dairy thermometer into milk mixture. With a large slotted spoon, stir milk 20 times with a gentle surface-to-bottom circular motion to evenly distribute heat. Let milk heat over high heat, covered and undisturbed, until its temperature reaches 192° to 194° (water will be boiling), 30 to 40 minutes; adjust heat to maintain temperature. If milk starts to get too hot, cool it in sink of water.
Step 2: Acidify the milk. Slowly pour vinegar over the warm milk. With a slotted spoon, stir milk 20 times with a gentle surface-to-bottom circular motion. Small curds will begin to form (they may have already). Cover pot with canner lid and let mixture stand, undisturbed, over high heat 25 minutes for curds to finish forming (temperature should remain between 192° and 194°; check occasionally and adjust heat as needed). Meanwhile, line mold with a double thickness of cheesecloth, trimmed to hang slightly over rim. Set a cookie or biscuit cutter in a medium bowl, then set mold on top.
Step 3: Drain your curds. Gently ladle curds and whey into mold, occasionally pouring out liquid from bowl, until draining slows down to a trickle. Smooth cheese in mold so it's level. Cover bowl and mold with plastic wrap and chill. Let curds drain in refrigerator until there are no visible pockets of liquid (curds should be moist), a few minutes to about 30 minutes more. Wrap mold and cheese tightly with plastic wrap (or, if you're using a colander, transfer cheese to an airtight container). Your ricotta is ready to eat. It keeps, chilled and wrapped airtight, up to 4 days. To serve, invert mold onto a plate or just spoon cheese out of container.
What you'll need:
These supplies may seem a bit mad-scientist, but they're easy to use. Find them—unless otherwise noted—at the Beverage People (thebeveragepeople.com or 800/544-1867). One very important note: Be scrupulously clean when making cheese—scrub surfaces with antibacterial soap and boil utensils (ladle, spoons, etc.) for 20 minutes before using. You don't want bad bacteria messing with the good.
Calcium chloride: A type of salt that helps firm up the curds.
Cheesecloth: A loosely woven cloth for lining cheese molds or colanders. Find at most grocery stores.
Ricotta mold: A small woven basket made of food-grade plastic; gives your ricotta a pretty shape.
Fromage blanc culture: Gives cheese both flavor and texture; looks a lot like freeze-dried yeast used for baking.
Dairy thermometer: Unlike a candy thermometer, measures low temperatures too. You can substitute an instant-read thermometer.
Vegetarian rennet: A lab-created version of the natural enzymes that coagulate milk.
Note: Nutritional analysis is per 1/4 cup.