"This recipe can be incubated in a self-cleaning oven. It will yield a nice, thick yogurt, as compared to some homemade yogurts that are runny. Save about 1 cup of this yogurt to use as your starter on the next batch."
Turn a self-cleaning oven on for 10 minutes at the lowest temperature, about 170 degrees F (77 degrees C). Turn the oven off and keep door closed to gently heat the oven walls, about 50 minutes.
Pour milk into a heavy pot over medium heat. Cook, stirring often, until an instant-read thermometer registers 185 degrees F (85 degrees C), about 15 minutes. Let cool to 115 degrees F (46 degrees C).
Mix 1 cup of cooled milk and yogurt together in a bowl until incorporated. Pour back into the pot of milk and stir to combine. Ladle the milk mixture into 5 very clean glass canning jars and screw on the lids.
Place the filled jars into the warm oven; leave the oven off and turn the oven light on. Leave the jars inside the closed oven for 6 hours. Remove the jars without shaking and place in the refrigerator until thickened, about 2 hours. Store in the refrigerator for up to 10 days.