Mix flour with 1 cup of the milk to smooth paste. Bring remaining milk to boil. Stir in flour mixture. Cook stirring constantly, until thickened and smooth. Gradually stir in buttermilk. Bring to boil remove from Heat. Place egg yolks in soup tureen. Gradually stir hot soup into the egg yolks. Serve hot with brown sugar on the side. Makes about 2 quarts.