Prepare Homemade Pancake batter: Combine milk and vinegar in a medium bowl; let stand for at least 5 minutes. (The mixture will thicken slightly and form small clumps.) Sift all the dry ingredients into a large bowl and whisk several times to distribute ingredients evenly. Combine yolks with melted butter in a small bowl and whisk to combine. Pour into milk mixture; mix well.In the bowl of an electric mixer fitted with the whisk attachment, whip egg whites until stiff peaks form. Pour wet ingredients into dry ingredients all at once and stir in the spinach; whisk until just mixed. Using a spatula, fold egg whites into the batter.