Prep: 15 min, Cook: 5 min, plus rising time. Sprinkle yeast over warm milk in a large mixing bowl. Stir in sugar and set aside 5 minutes. Beat egg yolks and vanilla in a small bowl. Stir into yeast mixture. Sift flour and salt together in a bowl and stir into liquid ingredients. Stir in melted butter and mix thoroughly. Carefully fold in beaten egg whites. Cover loosely and set aside to let rise in a warm place about 45 minutes, until mixture doubles in volume. Preheat waffle iron. When hot, lightly brush waffle iron surface with oil. Pour 3/4 cup batter per waffle onto hot iron. Close and bake 4-5 minutes or until iron stops steaming. Open cover and use fork to remove waffle. Continue with remaining batter. Serve immediately with strawberries and desired toppings or keep warm in 200°F oven until ready to serve.