Wash the leeks and cut them into thin half rings.
Disassemble the cauliflower into small inflorescences. A few small inflorescences must be preserved in order to be used in the future when serving soup.
In a saucepan, heat the butter and fry the leeks in it for 3 minutes. Add flour.
Cook, stirring, on low heat for about 3 minutes. Thus, a variant of the roux mixture is obtained, which is used as a thickener in many soups and sauces.
Carefully, in three steps, pour hot broth into a saucepan with Roux and, stirring vigorously, bring the mixture to complete dissolution in the broth. Bring to a boil and add the cauliflower. Cover and simmer for 30 minutes on low heat.
While the cabbage is cooking, prepare the inflorescences laid aside for decoration. Boil for 2-3 minutes in boiling water and transfer to a bowl with ice water. After that, they should be dried on a paper napkin.
When the cabbage becomes very soft, grind everything with an immersion blender (or in the bowl of a stationary mixer) until a completely homogeneous consistency is obtained.
Add salt to taste.
In a separate container, mix the cream and egg yolks, lightly whisk them with a whisk until smooth and add to the soup, continuing to whisk it with the same whisk.
Bring to a boil again, continuing to beat. The soup is ready!
Serve the soup immediately, garnished with blanched cauliflower buds and parsley leaves.