Pour the milk into a heavy pot and heat it gently until it reaches 90°C (195°F), stirring occasionally to prevent a skin from forming.
Remove the pot from heat and let the milk cool to about 43°C (110°F). If you don’t have a thermometer, test by dipping a cleaned pinky—if it’s comfortably warm (not scalding) after a quick count to 20, it’s close.
Dilute the 245 g (1 cup) of yogurt starter in a small amount of the lukewarm milk, then stir the mixture back into the pot until evenly combined.
Cover the pot and wrap it in a blanket or towel to keep it warm and undisturbed; leave it for at least 6 hours to incubate.
After incubation, transfer the covered pot to the refrigerator and chill without moving for 24 hours to set.
For traditional Greek yogurt, line a strainer with cheesecloth and strain the chilled yogurt for several hours until the whey stops draining; discard the whey or save for other uses.
Store the finished yogurt refrigerated; it will keep about ten days (or about one additional week if strained into Greek-style).
