First, lightly grease an 8x8-inch baking pan. Then preheat the oven to 350°F.

Add the egg whites to a large bowl.

Beat until stiff peaks form, then set aside.

Next, add the egg yolks and powdered sugar to another large bowl. Mix until smooth and well combined.

Then add the water and melted butter. Mix again until combined.

Next, pour in the lukewarm milk and vanilla extract. Add the flour and mix until the batter is smooth.

After that, gently fold the whipped egg whites into the thin batter. A few small fluffy pieces of egg white are fine.

Pour the batter into the prepared pan.

Bake for 45–55 minutes, or until the top is lightly golden and the center is mostly set.

Lastly, let the cake cool completely before slicing. Chill it for an even firmer custard texture if desired.