Cut the castella cakes into 7 mm sticks. Bake on the bottom rack in an oven preheated to 150℃ for about 12 minutes until they start to brown very lightly.
Turn off the oven, and leave in the oven for 10 minutes. Spread them on a rack, and when they have cooled, they become crispy! And done.
If you are using a store-bought castella cake, it contains a lot of sugar so it's easy to burn. In that case, bake in the oven for about 12 minutes at 130℃ - 140℃ while watching over them carefully.
Story Behind this Recipe
I often bake castella cakes at home as gifts, and I always have leftover crusts, so I made rusks.
These are sensitive to humidity, so after cooling them completely, wrap with a desiccant as gifts! The oven temperature and the baking time differ depending on the thickness and sweetness of castella cakes, so they are just guidelines!