Preheat oven to 320F (160C).

Grease a 8x8 inch baking pan and line it with parchment paper.

Separate egg whites and yolks.

Beat egg whites in a bowl until stiff and put aside.

Whisk egg yolks, vanilla extract and sugar until creamy.

Add melted butter and mix for half a minute.

Add the flour and unsweetened cocoa powder. Mix with a whisk until incorporated well.

Pour the milk gradually and continue whisking.

Add in the egg whites, one third at a time and gently stir with a whisk. Don't overmix, just stir until combined.

Pour this runny batter into the baking pan and bake for 60 minutes.

The centre of the cake will still be jiggly when removed from oven.

Let it cool for about half an hour and then remove from the pan. When it reaches room temperature, chill it at least for an hour.

Slice it in the size you like and dust with powdered sugar right before serving. You can garnish it with fresh fruit too.

Keep it in refrigerator until you finish it all.