Steamed sponge cakes live or die on the batter, and pumpkin makes that batter hard to read: its water content swings from one squash to the next, so the flour is added by feel, not by scale. You are after a thick paste that still stirs smoothly with no dry patches. Once it proofs to double, honeycombed with bubbles, it goes into oiled bowls and steams for twenty minutes.