Preheat a waffle iron. Separate the 3 eggs, putting the yolks in a large mixing bowl and the whites, plus the extra white, in another large mixing bowl. Whisk the yolks with the sugar and lemon juice until smooth. Add the oil in a thin stream, whisking until smooth, and then add the buttermilk, cream, lemon zest, and vanilla, whisking until combined. Sift the flour, baking powder, baking soda, and salt together into a bowl. Add the dry ingredients to the egg yolk mixture, whisking until fairly smooth, though a few lumps are fine. With a stand mixer fitted with a whisk attachment or a hand mixer, whip the egg whites until they hold soft peaks. Gently whisk one-third of the whites into the batter, and then carefully fold in the rest of the whites with a rubber spatula. Brush a little butter on the waffle grates, and add about 3/4 cup batter (or whatever will fill your waffle iron). Cook as directed. When done, spread the waffles with soft butter and serve with maple syrup.