Bring stock and 150 ml champagne to a boil. Stir 50 ml of whipped cream with the egg yolk. Add a little hot soup and stir this mixture into the soup. Do not make more. Season with pepper, salt, Worcestershire sauce and 1 pinch of sugar.
Cut slices of toast into hearts, toast in the melted butter-oil mixture until golden brown on both sides. Whip 125 ml of whipped cream until stiff and fold into the soup with 100 ml of champagne, seasoning if necessary. Serve with croutons and sprinkled with parsley.
Notes: Makes easily 3 servings.