Gently heat milk in a large lidded saucepan or Dutch oven to just below boiling, 200 degrees.
Remove the pan from the burner and let the milk sit until it is just hot to the touch, about 112 degrees. You can speed up the process by setting the pan in an ice bath.
Heat your oven to 115 degrees. Take a cup of the warm milk and whisk it with the yogurt. Add back to the milk.
Put the lid on the pan and wrap the whole thing in several layers of towels. (This insulates the pot and keeps the milk warm.)
Turn the oven off and place the pot on the middle rack. Set a timer for 4 hours. Try a spoonful. You’re looking for tart flavor and a creamy texture. Leave it longer for more tartness.
Once satisfied with the flavor, remove from oven and let cool completely. Transfer to smaller, airtight containers. Enjoy for two weeks.
Read the story + other recipes:: Edible Seasonals: Origins of Yogurt.