Put the milk into a thick-bottomed pan.
Whisk in the powdered milk if desired; it gives a better texture and thicker yogurt.
Heat the milk to 180° or to just below boiling point.
Cool the milk to 110° by placing the pan in a sink of cold water.
Rinse out the thermos with hot water. Pour most of the milk into the thermos.
Blend 1 teaspoon of natural yogurt with the remaining milk and add it to the thermos.
Twist the stopper on firmly and shake to mix well.
Leave for 4 - 6 hours or until firm.
Remove the stopper and put the thermos in the refrigerator to cool and set the yogurt.
You can use 1 teaspoon of this yogurt to "start" the next batch, but you will need a starter of fresh "live" yogurt every 7 - 10 batches.