Heat the milk in a Dutch oven over medium-high heat, stirring from time to time, until it reaches just below boiling point, about 180-190 F.
Remove from the heat and let the milk cool down for about 10 to 15 minutes, or until it is between 112-115 F.
In the meantime, preheat your oven for 5 minutes to the lowest temperature possible (around 110 F) and switch the oven light on. Then switch off the oven heat but leave the oven light on.
Dissolve the yogurt starter in a small amount of the cooled down milk.
Pour the yogurt starter into the Dutch oven and whisk in the vanilla extract and sugar until well combined.
Cover the pan with a lid and place in at the back of the oven with the oven light on for 10 to 12 hours without opening the oven. The longer you leave the yogurt in the oven, the thicker it will be.
Using a wooden spoon, check to see if the yogurt has reached the desired thickness and consistency. Once you are satisfied with the texture and taste of the yogurt, give it a gentle whisk to smooth the texture.
Transfer to an airtight glass jar and refrigerate for at least 4 hours.