Heat waffle iron to high. Heat oven to 200 degrees. In a medium bowl, whisk together the flour, sugar, baking powder, and salt; set aside.
In a medium bowl, whisk together egg yolks, milk and oil. Add to flour mixture, whisking just until large lumps disappear.
In a small bowl, beat the egg whites to stiff glossy peaks. Fold whites into batter.
Ladle about 1/3 cup batter into each section of the waffle iron; spread batter almost to the edges. Close iron and cook until no steam emerges, 5 to 7 minutes.
Transfer cooked waffles to a baking sheet. Transfer to oven to keep warm while baking the remaining batter. Serve with butter and maple syrup.