Traditionally, Castella cake is baked in a topless and bottomless wooden frame. With 4 sides insulated with wood, the batter is heated mainly from the top and bottom. This allows the cake to create its signature brown, flat and wrinkle-free crust.
I don't have a wooden mould, so I improvise. I put the pan holding the batter in a bigger pan, and there's corrugated cardboard tucked between the