Sift flour, baking powder, soda, salt. Combine buttermilk, eggs; add to flour mixture. With hand beater, or mixer at high speed, beat until smooth; whisk in butter. When waffle iron is ready to use, pour batter into center of lower half until it spreads about 1" from edges. Bring cover down gently. Cook until no steam escapes. Do not raise cover during baking. When waffle is done, lift cover; loosen waffle with fork; serve at once. Reheat iron before pouring in next waffle.