There are a few recipes out there, I chose one that was from a Japanese site, and the description said it tastes like the ones from Nagasaki, which is the castella capital of Japan.
4 med. eggs at room temperature (I used 4 lg and it still turned out well)
1 c. flour
Preheat oven at 350*F.
Mix together the milk and honey over LOW heat and bring it to body temperature. (I put them in a cup in a pan of water.)
Whisk the eggs (remove the white membrane by the yolk) in large bowl.
Add sugar, and beat 10 minutes.
While beating, add the milk/honey mixture, and beat 1-2 minutes.
Add the flour and beat 1-2 minutes.
Keep the mixer running and beating the ENTIRE TIME.
Line a 9x5 pan with parchment or wax paper.
Sprinkle a little raw sugar on the bottom.
Pour in batter, no more than 4/5 full.
Tap the pan to let the air bubbles out.
Bake for 1 minute. "Cut" the top of the batter with a spatula to get rid of air pockets. Return to oven and repeat 3 times total. Bake 10 minutes more.
Lower oven temperature to 310*F, and bake 45 minutes. (see notes below)
Remove from the pan while still warm, wrap in saran wrap, let cool, then refrigerate 1-2 days.
***Some changes I would make the next time. I had lined the entire pan with parchment paper. Next time I would just do the bottom. Having reading other recipes, the cake pulls from the sides when baked, so mine pulled the paper with it and it was hard to peel off without ruining some of the cake. Initially the recipe said to bake 25-35 minutes. After which I poked it with a skewer and the entire thing deflated and made a well in the middle. In times like this, I think overbaking is better than underbaking and then poking it. After putting it back in the oven a couple times, my total baking time after lowering the temperature was 45 minutes. Next time I'd try a 8x8 or 9x9 brownie pan, and stick to the original times of 25-35 minutes. Overall, I am happy with the recipe, it was easy and very tasty. And this recipe uses only 4 eggs, while others use 7 or 8 (maybe more eggs make it fluffier?) Sometimes seeing a recipe using too many eggs makes me put off making it for a while due to the high cost of cage free eggs. Maybe after I have my own hens I'll try those other recipes. More castella info and a different recipe can be found at Just Hungry.