Sprinkle yeast over warm milk; stir to dissolve. Beat egg yolks and add to yeast mixture with vanilla. Sift together flour, sugar and salt; add to liquid ingredients. Stir in melted butter and combine thoroughly. Beat the egg whites until stiff; carefully fold into the mixture. Let stand in a warm place about 45 minutes until mixture doubles in bulk. Use 7/8 cup mix per waffle.