Helen Goh offers a springtime spinach sponge cake recipe that captures the season's pale, luminous green. The cake uses spinach to achieve a vivid color that softens in the oven, losing any metallic edge and leaving behind a gentle, vegetal brightness. Baked in a shallow tin and finished with cream cheese icing, the cake produces an ideal ratio of cake to frosting when sliced into squares. The combination of tender cake and tangy icing delivers an uncommonly good result suited to spring entertaining.