1 liter of raw milk (store bought whole milk also works)
2 tbsp of natural, unsweetened yogurt
Heat the milk in a pan over medium heat to just below boiling, stirring occasionally to avoid scorching.
Remove from the heat and let the milk cool down to around 45ºC (or until you can keep your finger in it while you count to 8).
In small bowl, combine a cup of warm milk with the 2 tbsp of yogurt and mix until the yogurt has dissolved into the milk.
Pour the thinned yogurt into the warm milk and whisk very gently (you just want to spread the thinned yogurt all over the warm milk).
Cover the pan with a lid and wrap it in a blanket. Let the yogurt set undisturbed for at least 6 hours (or overnight) in a warm place without any currents of air.
Transfer the yogurt to a container and put it in the fridge to cool down. It will keep for up to 2 weeks.