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 3 cm 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.