Grease and line an 8×8-inch (20x20cm) baking pan with parchment paper. Leave an overhang of parchment paper making it easy to lift out the cake after baking

Separate the eggs. Yolks in one bowl and whites in another.

Add sugar and vanilla extract to the bowl with the yolks. Beat until the mixture turns pale and creamy (about 4 to 5 minutes)

Pour in the cooled melted butter and mix for 1 minute on low speed.

Sift in the flour and unsweetened cocoa powder. Beat or whisk to combine.

Slowly pour in the milk while beating the mixture and until just combined.

Set the bowl aside.

Preheat the oven now to 325° F / 170°C (150°C if you are using a fan assisted oven)

Rinse out and dry the whisk or beaters (if you were using for the yolk mixture)

Beat the egg whites on medium speed for 1 minute. Then add in the 1 Tablespoon white granulated sugar and salt.

Continue beating for about 4 minutes. When the beaters are lifted from the bowl, the whites should now form stiff peaks. Be careful not to overbeat the egg whites past this stage. Or the result will be curdled egg whites.

Use a spatula to transfer the beaten egg whites to the yolk mixture. Fold the beaten whites (½ of the mixture at a time) gently into the yolk mixture with a spatula.

Don't worry if you see a few specks of egg white on the surface. You don't want to overbeat it. The mixture will look thin and quite runny even as you pour it into the baking pan. This is also perfectly normal.

Pour the mixture into your prepared pan and bake in the preheated oven for 40-45 minutes. see notes

If the top is browning too quickly before its baked, then shape foil into a dome and place it over the cake. It should not touch the batter.

Check the cake at 40 minutes. The top of the cake should be firm to the touch.

To make sure that its baked, very gently, shake the tin. There should be no movement of batter beneath the top of the cake. If there is, simply place it back into the oven, cover with foil (so that the top does not burn). Check on it every 5 minutes until firm.

Once baked, turn the oven off. Leave the cake in the oven for 5 minutes with the door slightly open. This prevents the cake from collapsing too harshly due to the sudden temperature change. You can wedge a wooden spoon between the door which will help keep it open.

Remove the cake from the oven, leave it in the pan to cool for about 30 minutes.

Leaving the cake in the pan, refrigerate for 1 hour, allowing the custard to set. The cake will shrink down slightly as it cools, and this is normal

Remove the cake from the fridge, sift cocoa powder or powdered sugar over the top if you like. Use the overhang of the parchment paper to lift the cake out of the pan. Then slice into portions and serve. (See notes)