Melt the butter in a saucepan and stir in the flour. Cook, stirring, for 2 minutes, then gradually stir in the stock and milk. Bring to the boil and simmer, still stirring, until thickened. Cool slightly, then strain the soup and return it to the cleaned pan. Add dalt, pepper and the egg yolk and cook very gently, stirring, until the soup thickens.