Combine first 5 ingredients in a large bowl; make a well in center of mixture. Combine milk, yogurt, and oil; stir with a wire whisk. Add to dry ingredients, stirring just until dry ingredients are moistened.
Beat egg whites and cream of tartar at high speed of an electric mixer until stiff peaks form; gently fold beaten egg white mixture into batter.
Coat an 8-inch square waffle iron with cooking spray; allow waffle iron to preheat. For each waffle, spoon 1 1/4 cups batter onto hot waffle iron, spreading batter to edges. Bake 4 to 5 minutes or until steaming stops. Repeat procedure with remaining batter. Cut each waffle into 4 squares. Top with maple syrup, if desired.