Begin by preheating your oven to 325°F (165°C). Line an 8×8-inch baking dish with parchment paper, ensuring that the parchment extends over the sides for easy removal later. Lightly grease the parchment with cooking spray to prevent sticking, then set the prepared dish aside.

In a clean, dry mixing bowl, beat the 4 egg whites using an electric mixer on medium speed until stiff peaks form. This means the egg whites should hold their shape firmly when the beaters are lifted. Once whipped, set the egg whites aside.

In a separate large mixing bowl, combine the 4 egg yolks with ¾ cup (150 g) of granulated sugar. Using an electric mixer, beat the mixture until it becomes pale yellow and creamy, indicating that the sugar has dissolved and the yolks are well incorporated.

Gradually add ½ cup (113 g) of unsalted butter, which has been melted and slightly cooled, to the egg yolk and sugar mixture. Pour in 1 teaspoon of vanilla extract as well. Mix thoroughly until the ingredients are evenly combined, ensuring a smooth consistency.

Sift in ¾ cup (95 g) of all-purpose flour to the mixture. Stir the flour in gradually, mixing until it is fully incorporated and the batter is smooth without any lumps.

Pour in ¼ cup (60 ml) of freshly squeezed lemon juice and add the grated zest from 2 large lemons. Stir the mixture until the lemon juice and zest are thoroughly combined, infusing the batter with a bright, citrusy flavor.

Gradually pour in 1 ¾ cups (414 ml) of lukewarm milk while continuously stirring the batter. This ensures that the milk is well incorporated and the mixture remains smooth and free of lumps.

Gently add the previously whipped egg whites to the batter by whisking them in by hand. It is not necessary to fold them in completely; leaving small lumps of egg whites is acceptable. These lumps will rise to the surface during baking, creating a light and airy top layer. Note: The batter will be very thin, resembling a custard in consistency.

Carefully pour the batter into the prepared baking dish. Place the dish in the preheated oven and bake for 40-60 minutes. The cake is done when it is barely jiggly in the center and the top feels firm to the touch. Baking times may vary depending on your oven and the type of pan used, so begin checking for doneness around the 35-minute mark. If the top starts to brown too quickly, loosely cover the cake with aluminum foil to prevent over-browning. Be cautious not to overbake, as this can result in a rubbery texture rather than a creamy, custardy center.

Once baked, remove the cake from the oven and allow it to cool completely in the baking dish. After cooling, dust the top with powdered sugar for a delicate finish. Slice the cake and serve it chilled or at room temperature. Any leftovers should be stored in the refrigerator to maintain freshness.