Similar to chiffon cake, but less sweet and with a slightly bouncier, leaner crumb, this is the type of plain, unadorned sponge cake sold in Chinese bakeries. Often called Hong Kong cupcakes and baked in paper wrappers in individual cone-shaped portions, the cakes are eaten out of hand with coffee or tea as a breakfast or snack. We bake our version in a springform pan with a parchment liner that results in charmingly uneven edges. The top domes during baking, then deflates while cooling, leaving a wrinkled surface. We flavor our cake with vanilla and add extra yolks for richness. It’s best the day of, but leftovers will keep wrapped in plastic in the refrigerator for up to three days; bring to room temperature before serving.