In a bowl, mix the three dry ingredients with a fork.
In a second, larger bowl, beat the egg yolks. Add the milk, and whisk in the melted butter. Stir in the vanilla.
Pour the dry ingredients into the wet ingredients. Stir with the whisk until almost blended, but leave lumps in the batter. The batter will be very thick, but if it won't pour at all add a little water.
In a clean bowl with a clean whisk or beater, whip the egg whites with the cream of tartar into a froth until it forms soft peaks. Spoon this into the batter and, with a wooden spoon, fold it carefully into the batter, just enough to mostly blend.
Pour the batter into the waffle iron and cook till crisp. Serve with pure maple syrup.