Magic Custard Cake Loaf

Prep: Preheat your oven to 160°C (325°F). Grease a standard 9×5 inch loaf pan and line it with parchment paper, leaving an overhang on the sides for easy removal.
Whip Egg Whites: In a clean bowl, beat the egg whites until stiff peaks form. Set aside.
Mix Yolks and Sugar: In a separate large bowl, beat the egg yolks and sugar until the mixture is pale and creamy (about 2 minutes).
Combine Fats and Aromatics: Stir the melted butter and vanilla into the yolk mixture until smooth.
Add Flour and Milk: Mix in the flour until just combined. Gradually whisk in the lukewarm milk until the batter is thin and smooth—it will look very watery, similar to crepe batter.
Fold in Whites: Gently fold the stiff egg whites into the batter in three additions. Note: Do not over-mix; you want small “islands” of egg white floating on top to create the sponge layer.
Bake: Pour the batter into the prepared loaf pan. Bake for 50–60 minutes. The cake is done when the top is golden and the center has a slight jiggle but is not liquid.
Cool and Set: Allow the cake to cool completely in the pan at room temperature, then refrigerate for at least 2–3 hours (or overnight) to allow the custard layer to set before slicing.

Temperature Matters: Ensure your milk is lukewarm, not hot, to prevent curdling the eggs.
Don’t Rush the Chill: Slicing the cake while warm will cause the creamy custard layer seen in your photo to run.
Avoid Over-folding: If you fold the egg whites in too thoroughly, you will lose the distinct spongy top layer.