Clear-Conscience Waffles

Stir together whole-wheat flour, all-purpose flour, baking powder, salt and baking soda in a large bowl. Whisk together buttermilk, egg yolk, vanilla (if using) and oil in a separate bowl. Add to the dry ingredients and stir with a wooden spoon just until moistened. Beat the 3 egg whites in a grease-free mixing bowl with an electric mixer until soft peaks form. Add sugar and continue beating until stiff and glossy. Whisk one-quarter of the beaten egg whites into the batter. With a rubber spatula, fold in the remaining beaten egg whites. Preheat waffle iron. Brush the surface lightly with oil. Fill the iron two-thirds full of batter. Cook until the waffles are crisp and golden, 5 to 6 minutes. Repeat with the remaining batter, brushing the surface with oil before cooking each batch.