For the rice gruel soup, wash rice well under running water and drain again. Pour water or soup into a pot, add rice for rice gruel soup and cook until tender, depending on the variety, at least 20 minutes (longer is better). Strain and bring to a boil again in the pot. Season with salt as needed and add the butter.
If the soup is not served as a diet meal, whisk the egg yolks with the whipping cream and stir into the soup, which is no longer boiling strongly.
However, as a diet meal for the sick, both cream and yolk and butter should be omitted.