In a large saucepan, heat the butter and in it cook the cauliflower and leeks over moderately low heat for 5 minutes, stirring constantly.
Add stock, bring to a boil, lower the heat, and simmer, covered, for 25 minutes.
Purée the vegetables in a food mill or food processor and return to the saucepan. Whisk in the milk, ricotta cheese, nutmeg, salt and white pepper to taste, and Parmesan and bring to a simmer, stirring constantly. Remove from the heat.
In a small bowl, beat the egg yolks into the heavy cream. Whisk in 1 ladleful of the soup. Whisk this mixture into the soup and heat over low heat, stirring, until the soup thickens. Do not allow the soup to boil or the eggs will curdle.
Remove from heat and stir in the softened butter. Serve in heated bowls and garnish with chopped fresh parsley.