Combine the dry ingredients. Mix together the buttermilk, sour cream or yogurt and the egg yolks. Stir in the butter and vanilla.
Brush the waffle iron lightly with oil and preheat it. Stir the wet into the dry ingredients. Beat the egg whites with the whisk or electric mixer (spotlessly clean ones work best) until they hold soft peaks. Stir them gently into the batter.
Spread a ladleful or so of batter onto the waffle iron and bake until the waffle is done, usually 3 to 5 minutes, depending on your iron. Serve immediately or keep warm for a few minutes in a low oven.