In a medium saucepan over medium heat, whisk together the egg yolks, sugar, and cornstarch until smooth. Gradually add the milk while whisking continuously. Cook for 5–7 minutes, stirring constantly, until the mixture thickens. Remove from heat, stir in the vanilla extract and a pinch of salt. Set aside to cool slightly.

Set your oven to 325°F (160°C). Lightly grease an 8-inch square baking dish or line it with parchment paper for easy removal later.

Melt the butter and allow it to cool slightly so it doesn’t cook the eggs when mixed. Set it aside.

Carefully separate the egg yolks from the whites. Place the yolks in one mixing bowl and the whites in another (make sure no yolk gets into the whites).

Add the caster sugar to the bowl of yolks and beat until pale and fluffy. Mix in the tablespoon of water and the vanilla extract. Gradually pour in the melted butter while mixing continuously.

Slowly mix in the sifted flour until just combined. Then gradually add the lukewarm milk while whisking to keep the mixture smooth. Once incorporated, gently fold in the prepared custard layer. The batter will be quite liquid this is normal.

Using a clean whisk or electric mixer, beat the egg whites until stiff peaks form. This gives the cake its signature layered texture.

Gently fold the whipped egg whites into the batter, a third at a time. Don’t over mix it’s fine to see a few lumps or white streaks.

Pour the mixture into the prepared baking dish. Bake for 50–55 minutes, or until the top is golden and slightly firm. The center may jiggle a bit that’s okay; it sets more as it cools.

Let the cake cool completely in the pan. Once cooled, refrigerate for at least 1–2 hours (overnight is best). Dust with powdered sugar before slicing. Serve chilled or at room temperature.