Beat eggs, water and sugar until
thick and creamy, 10 minutes with a mixer at high speed or 20
minutes by hand.
Fold in flour, then melted butter. Pour
batter into pan. Fill bottom of wok with water and bring to
a rolling boil.
Place steamer rack in wok; set cake pan on rack.
Cover. Reduce heat to medium and steam until cake begins to
pull slightly from pan and a wooden pick inserted in cake comes
out clean, about 20 minutes. Turn out cake; peel off parchment.