Sift flour and salt together, make a well in the centre, and into this pour the egg yolks and cool melted butter; stir briskly, drawing the flour in by degrees and adding the white wine, cider or ale, a little at a time. Let it stand for 1 hour.
When ready to cook, stir in the milk or water and fold in the stiffly beaten egg whites. Wash the mushrooms and sprinkle with the lemon juice and seasoning. Dip each mushroom in the batter and fry in hot oil until golden. Drain well and serve very hot.