In a large saucepan, bring 1 1/2 inches of water to a boil. Set a large stainless steel bowl over the saucepan and add the milk; do not let the bowl touch the water. Turn the heat to low and gradually heat the milk, whisking, until it registers 180° on a candy thermometer. Keep the milk at 180° for 30 minutes, adjusting the heat as necessary.
Remove the bowl from the saucepan and let the milk cool down to 106°, stirring often. Meanwhile, preheat the oven to 110°.
If using yogurt, whisk it with 2 cups of the warm milk in a bowl until smooth, then add it back into the warm milk.
If using powdered yogurt culture, sprinkle the powder all over the warm milk.
Whisk the cultured warm milk for 3 minutes. Fill several clean jars to 1 inch below the rim with the cultured milk. Cap the jars and place in the warmed oven (or a yogurt maker or other gently heated spot); the cultured milk should stay between 105° and 110° during the entire process. Begin checking the yogurt after 4 1/2 hours; it’'s ready when it is thick, tangy and surrounded by a small amount of clear whey. If using a pH meter, the yogurt is ready when it registers 4.5. Depending on how active the cultures are, it can take up to 18 hours for the yogurt to set and develop its characteristic tang. Refrigerate until thoroughly chilled or overnight.
Line a large mesh colander or strainer with a moistened cotton cloth or several layers of cheesecloth and set it over a large bowl. Scoop the yogurt into the colander. Cover with plastic wrap and refrigerate for about 6 hours, or until it reaches the desired thickness.