In a large bowl sift to combine the flour, baking powder, 2 tablespoons of the sugar and salt. In small bowl whisk the egg yolks until smooth, then add the milk, vegetable oil, lemon extract, and lemon zest. Make a well in the center of the dry ingredients. Pour in the liquid ingredients and gently stir the batter, with a wooden spoon, until just combined. Be careful not to overmix. You can use a hand beater or a stand mixer on low speed, if desired.
In a very clean bowl, whisk the egg whites until foamy. Add the remaining sugar and beat until soft peaks form. Gently fold the whites into the batter. Pour about 1/4 cup to 1/2 cup batter into an oiled waffle iron, sprinkle with walnuts, then close the iron. Cook until golden brown, about 5 minutes, depending on waffle iron settings. Transfer to a serving platter and keep warm in a low oven. Repeat with remaining batter. Serve warm with maple syrup.