Sift 2 cups flour into a large bowl (set aside 1 cup for dipping the onion rings). In a separate bowl, whisk egg yolk and mix in beer, butter and salt. Slowly stir the egg yolk mixture into the flour and mix well. Allow the mixture to stand about 40 minutes. Heat deep fryer to 350°F. In a small bowl, beat the egg white until stiff peaks form. Gently fold egg white into the batter. Coat each onion ring with remaining flour and dip into batter; deep fry until golden coloured. Drain on paper towels or brown paper and serve. Serves 6.