Preheat your oven to 150°C. Line a 6-inch cake pan with parchment paper, and set it aside.

Prepare a larger tray to create a water bath for the cake.

Crack the 4 eggs into a clean, dry bowl, separating the egg whites from the yolks carefully. Place the yolks in a large bowl and set aside the egg whites (make sure the egg white bowl is clean and dry).

Heat 40g of corn oil in a pan over low heat. Once the oil starts to lightly smoke and you see fine lines at the bottom of the pan, remove it from the heat.

Quickly pour the hot oil into a heatproof bowl. While the oil is still hot, sift in 40g of all-purpose flour and 10g of cornstarch. Stir quickly with a spatula until smooth and lump-free.

Add 70g of milk and continue to mix until smooth. Let the mixture cool down slightly until it's just warm to the touch. Once cooled to the right temperature, add the 4 egg yolks and stir to combine.

In a separate clean bowl, add a few drops of white vinegar or lemon juice to the 4 egg whites. Add 50g of white sugar.

Whisk the egg whites at low speed until the sugar dissolves, and large bubbles start to form. Increase to medium speed and continue to whip until soft peaks form (you should see a slight curve when you lift the whisk). Stop when the egg whites reach this stage.

Gently fold one-third of the whipped egg whites into the egg yolk batter using a spatula. Mix from the bottom up, being careful not to deflate the egg whites.

Once the first batch of egg whites is fully incorporated, pour the mixed batter back into the remaining whipped egg whites. Gently fold from the bottom up with a spatula until the batter and egg whites are fully combined and there are no visible layers. Be careful to fold lightly to avoid deflating the egg whites.

Pour the batter into the prepared 6-inch cake pan. Gently tap the pan on the counter to release any large air bubbles.

In the larger pan, pour 2-4 cm of warm water. Place the pan with the batter into this larger pan (this is the water bath method).

Place the setup into the preheated oven. Bake at 150°C for 20 minutes, then lower the temperature to 130°C and bake for another 50 minutes. Avoid opening the oven door during baking to maintain an even temperature.

When the cake is done, carefully remove the pan from the oven using oven mitts. Invert the cake onto a cooling rack and let it cool naturally.

Once completely cooled, peel off the parchment paper. Slice the cake into pieces, and enjoy the soft, melt-in-your-mouth Castella cake!