Prepare the pan: Grease an 8×8‑inch (20×20 cm) baking pan. Line it with parchment paper, leaving an overhang on the sides so you can lift out the cake later.

Separate the eggs: Put the yolks into one bowl, the whites into another.

Beat egg whites: With a mixer, beat the egg whites on medium speed for ~4 minutes, until stiff peaks form. Be careful not to overbeat, as that can cause graininess. Set aside.

Make yolk mixture:

Whisk together the egg yolks and sugar until pale and creamy (about 3 minutes).

Add the melted butter and vanilla extract, mix until combined.

Fold in the flour gently, trying not to overmix.

Stir in lemon juice and lemon zest.

While beating, slowly pour in the lukewarm milk and mix until everything is fully incorporated.

Preheat the oven: to 325°F (170°C). If your oven is fan‑assisted, 150°C should work.

Combine mixtures: Gently fold the whipped egg whites into the yolk/flour/lemon/milk mixture. Use a spatula, not a mixer. Fold carefully—don’t worry if you see a few white streaks; that’s normal.

Bake: Pour the combined batter into the prepared pan. Bake for ~40‑45 minutes, or until the center is barely moving and the top feels firm to the touch. (Ovens vary—start checking around 30‑35 minutes). If the top is browning too fast, tent foil loosely over the top.

Cooling and chilling: Remove from oven, leave cake in pan for 20 minutes. Then refrigerate the cake (still in the pan) for at least 1 hour. This helps the custard layer set and makes slicing cleaner.

Serve: Lift the cake out using the parchment overhang. Dust with powdered sugar, slice, and serve. You can serve cold or at room temperature depending on your texture preference: colder = firmer custard; warmer = creamier.