Preheat the oven to 325°F (165°C). Line an 8×8-inch baking dish with parchment paper, allowing the paper to overhang the sides. Lightly grease with cooking spray and set aside.

In a clean mixing bowl, whip the egg whites until stiff peaks form. Set aside.

In a separate bowl, beat the egg yolks and granulated sugar until pale yellow and thickened.

Add the melted butter and vanilla extract to the egg yolk mixture and mix until fully incorporated.

Gradually add the flour, mixing until smooth and evenly combined.

Stir in the lemon juice and lemon zest until incorporated.

Slowly add the lukewarm milk, mixing until fully combined. The batter will be very thin.

Gently whisk the egg whites into the batter by hand. Do not fully incorporate; small lumps of egg white should remain visible. These will form the cake’s top layer during baking.

Pour the batter into the prepared baking dish. Bake for 40–60 minutes, or until the top is set and lightly golden while the center remains slightly jiggly. Begin checking at 35 minutes. If the top browns too quickly, loosely cover with aluminum foil.

Remove from the oven and allow the cake to cool completely in the pan. Dust with powdered sugar before serving.

Store leftovers in the refrigerator.