Fill a pot with the stock and put on the fire. When it is about to boil, add the rinsed rice and cook over low heat for 20 minutes. Beat the yoghurt, egg yolks and lemon juice together and add to the water-rice mixture. Continue mixing until it returns to a boil, then add the salt. Melt the butter in a skillet, add the dried mint and brown slightly. Pour half of it into the boiling soup. Drizzle the remaining half over the soup in the tureen. Serve piping hot.