Prewarm an oven to 110F. Once the oven comes to temperature, turn it off and minimize opening the door to retain the heat as much as possible.

Heat milk in a large saucepan over medium-high heat, stir frequently

Turn off the heat just before the temperature registers 180°F on an instant-read or candy thermometer. (Do not leave it unattended—it can boil over very quickly.). Continue to monitor the temperature to ensure it does not go over 180F

After 5 minutes at 180F, take the milk off the stove and pour into a clean, heat-safe container. Let stand, stirring frequently, until cooled to 110°F

Transfer a small amount of the 110°F milk into a small bowl and mix in the yogurt or whey, then stir this mixture back into the rest of the milk. Mix gently but mix well.

Cover the container with cling film, poke some small holes in the cling film, and wrap it in a clean kitchen towel to help keep it warm. Place in the prewarmed oven and let stand, undisturbed for 8-12 hours until thickened and tangy.

Once the yogurt has set, refrigerate for about 2 hours. The yogurt will thicken a bit more in the refrigerator. Stop here if you just want regular yogurt. Continue to the next step if you want to turn your yogurt into Greek yogurt

Line a large fine-mesh sieve with 2 layers of cheesecloth or coffee filter paper and place over a large bowl. Spoon the cooled yogurt into the cheesecloth, then cover and refrigerate for 8 to 12 hours, depending on how thick you want it.

Store the finished yogurt in the refrigerator. Mix in some of the whey to adjust the texture as desired. Save the whey in a separate air tight container