Bring the broth to a boil and season with salt to taste, then cook the pasta in it. When the pasta is done, remove the pan from the stove.
Mix yoghurt, egg yolk and flour in a bowl so that you get a smooth mass without lumps. Remove a ladle from the soup, add to the yoghurt mixture and stir so that the yoghurt mixture becomes a little runny and gets a good temperature, otherwise the yoghurt mixture may stagnate and flakes form. Now stir the liquid yogurt mixture quickly into the soup.
Melt the butter separately in a small pan and toss the mint in it for a minute. Finally add the mint butter to the soup and stir well. If necessary, heat the soup again, but do not let it boil, otherwise unsightly lumps can form.