1 liter full-fat milk
2 -3 tablespoons plain yogurt (room temperature) or 2 -3 tablespoons of home-made yoghurt, mixed with a few tablespoons of milk, to thin (room temperature)

Bring milk to just under boiling point and then pour the milk into a glass or earthenware dish. Let the milk cool to about 42C/104F Pour the yogurt/milk mixture into the milk carefully without disturbing the skin that may have formed on the surface of the milk. Cover with a cloth, place in a warm, draft-free place for 8 to 12 hours or overnight, and do not disturb it until the yoghurt thickens. Drain any excess liquid and store in the fridge.

To make a "thick" yoghurt, remove the skin on the surface of the yoghurt (this is edible, btw, and lovely sprinkled with a bit of sugar) and pour the yoghurt into a muslin or cheesecloth bag. Hang the bag over a bowl and let drain for about 2 hours or until the desired thickness is obtained.

Note: I have now made this recipe using 2% milk and have achieved very good results. Obviously, richness is lost, but so are calories (and fat). .