Preheat your oven to 325°F (165°C). Line an 8×8-inch baking dish with parchment paper, leaving some overhang on the sides for easy removal. Lightly grease the parchment paper with cooking spray and set aside.
In a clean mixing bowl, whip the egg whites until stiff peaks form, then set aside.
In a separate large mixing bowl, beat the egg yolks and sugar together until the mixture is pale yellow and creamy.
Add the melted butter and vanilla extract to the egg yolk mixture, mixing until evenly combined.
Gradually mix in the flour until fully incorporated.
Add the freshly squeezed lemon juice and grated lemon zest, and mix until well combined.
Slowly beat in the lukewarm milk until the batter is smooth and well combined. The batter will be very thin.
Gently whisk the stiff egg whites into the batter by hand. It’s okay to leave small lumps of egg whites in the mixture—these will float to the top during baking and create a light, airy top layer.
Pour the thin batter into the prepared baking dish.
Bake for 40-60 minutes, or until the cake is just barely jiggly in the center but the top is firm to the touch. Baking time may vary depending on your oven and baking dish, so start checking the cake at around 35 minutes. If the top begins to brown too quickly, cover it loosely with aluminum foil. Be careful not to overbake, as this will result in a rubbery texture instead of a custardy center.
Allow the cake to cool completely in the baking dish. Once cooled, dust the top with powdered sugar before serving.
Store any leftovers in the refrigerator.