Grease a 15 x 25 cm cake tin.
Separate the egg whites from the yolks into two separate bowls.Beat the egg whites until stiff peaks form using an electric mixer. Set aside. Beat the egg yolks with the sugar until creamy.Add the melted butter and beat again for 1 to 2 minutes. Stir in the flour and mix gently until the batter is smooth.
Add the lemon juice and zest, then pour in the warm milk, stirring well.
Preheat your oven to 160°C (325°F).
Gently and gradually fold in the beaten egg whites using a spatula.Mix roughly. Small lumps should remain on the surface. Pour the mixture into your baking pan and bake for about 45 minutes, until the surface is golden brown.Baking time may vary slightly depending on your oven; you may need to extend or reduce the baking time by 5 minutes. Then let it cool completely before refrigerating for at least 2 hours.