Extremely popular in Japan, Castella cake is rather unusual. It's always square or rectangular, never round, because traditionally it's made in a bottomless wood frame. The crust, both top and bottom, is perfectly flat, unwrinkled, thick and well-browned. The cake is made with bread flour but it's soft and practically "poreless". How do you get all these characteristics without cheating by adding oil or a man-made emulsifier? And without a wood frame? Just follow my recipe.