Castella cake is typically baked in a water bath. The generated steam gives a moist and fine crumb, as the cake rises slowly and a smooth surface can be achieved without cracks. If your cake pan is one piece without gap or joint, you can place cake pan directly into a bigger pan (as a water pan). For me, my cake pan has folding joints, so I place it into a bigger pan which then place into a bigger water pan. I prefer this way over using the aluminium foil to wrap my cake pan. *Recipe* 5 egg yolk