In a large mixing bowl, use a whisk to sift together flour, sugar, baking powder and salt.
In a separate bowl, whisk together milk, egg yolks, butter and vanilla. Add to dry ingredients mixing just until fully moistened.
Using an electric mixer on high speed, beat egg whites with cream of tartar until stiff peaks form. Gently fold into batter. Batter will be lumpy.
Spray both sides of the preheated waffle iron with nonstick cooking spray. Pour in enough batter to lightly cover the surface of the iron, about ½ to ¾ cup, depending on size.
Close iron cooking until waffles have crisp and golden exteriors, about 5 minutes or according to manufacturer recommendation.
Cook waffles in batches. Repeat spraying the waffle iron with cooking spray as needed. Keep waffles warm between batches in a 200°F oven in a single layer on a sheet pan, until serving.
Serve immediately with butter or topped with fresh berries, a dusting of powdered sugar, drizzled with maple syrup and whipped cream.