Preheat a standard-sized waffle iron.
In a large mixing bowl, sift together the flour, baking powder, baking soda, and salt.
In a medium-sized bowl, beat the egg yolks and stevia together well. Add the buttermilk and butter, and continue to beat until well combined.
Add the liquid mixture to the dry ingredients and mix together with a few swift strokes. Do not overmix.
Using an electric hand mixer, beat the egg whites until stiff, then fold them into the batter.
When the iron is ready, add enough batter to cover about two-thirds of the grill's surface and close the lid. Cook about 4 minutes, or until steam stops escaping from the sides of the iron. Lift up the cover and remove the waffle.* Repeat with the remaining batter.