For key visual step-by-step photos, refer to the body of the post.

Prep:

• Preheat oven to 250°F/121°C.
• Line a piece of parchment paper at the bottom of the 6" round cake pan.
• Separate the egg whites from egg yolks (while cold).

Make the egg yolk batter:

• Sift cake flour into a medium mixing bowl. Set aside.
• Pour 30ml oil in a small pot and heat until the oil reaches 75°-80°C.
• Add the flour mixture into the hot oil and mix well with a spatula.
• Pour cold milk and vanilla extract into the flour mixture to reduce the temperature and mix well.
• Add all the egg yolks in at once, and mix until very smooth. Set aside.

Whip the egg whites:

• Place the egg whites in a stand mixer fitted with whisk attachment.
• Whip egg whites, and add in cream of tartar when it becomes foamy.
• Gradually add in sugar and whip until soft peaks.

Combine the mixtures:

• Fold in egg whites into the yolk flour mixture in 3 batches, until well combined with no white streaks.
• Pour cake batter at a height of 8-10" to break any air bubbles, and into the baking pan/baking tray.
• Tap the pan a few times on the counter to release any remaining bubbles.
• Place cake pan into an aluminum foil dish/baking pan and place into a roasting pan in the centre of the oven.
• Pour in body temperature tap water (35°C) into the roasting pan to ½" deep.

Bake:

• Bake the castella cake at 250°F/121°C for 60 minutes.
• Increase the oven temperature to 300°F/149°C and bake for an additional 15 minutes.
• Test for doneness by gently tapping on the cake with a silicone spatula. If it feels springy, it is done. If not, extend the time for 5 more minutes.
• Turn off heat.
• Remove water bath and leave cake in the oven for another 5 minutes to dry up.
• Take the cake out of the oven and cool on a wire rack for 10 minutes.
• Give the cake a good tap around the edges to loosen.
• Remove bottom from the pan and cool completely on a rack.
• Cover and chill the cake or slice and serve.