Whisk together first 4 ingredients in a large bowl. Whisk together egg yolks and buttermilk in a small bowl. Beat egg whites and cream of tartar at high speed with an electric mixer until stiff peaks form. Using a fork, stir yolk mixture into flour mixture just until dry ingredients are moistened. Stir in butter and vanilla. (Batter will be lumpy.) Fold in egg white mixture just until incorporated. (Do not overmix.) Pour about 1/2 cup batter for each waffle into a preheated, oiled Belgian-style waffle iron; cook according to manufacturer's instructions until waffles are golden brown and crisp. Keep warm in a single layer on a baking sheet in a 200° oven up to 30 minutes. Serve with syrup.