Preheat oven at 150°C/300°F.

In 2 large bowls, separate the egg whites and egg yolks. Set the egg whites aside.

In the egg yolks bowl, add the sugar. Mix well until well combined. Mix about 1 min so that the sugar is partially dissolved or dissolved.

Add the oil and milk. One at a time and mix one at a time.

Sift in the dry ingredients. Fold in using a spatula until just until the flour disappear. Do not over mix.

In the other egg whites bowl, add in the cream of tartar. Mix it until bubbly or foamy. Then gradually add in the sugar, little by little. Mix until stiff peaks formed.

Add the egg white mixture into the chocolate batter. Use a spatula to gently fold in the egg whites. Switch to a whisk and continue folding in a circular motion. This method will help in elevating the cake later. Do it gently. However, do not over mix at this point. Mix just until the whites are gone.

Grease a 7-inch square pan. (you can also use a 7- or 8-inch round pan if you desire). Line 2 parchment papers as shown in the video to cover the bottom and all 4 sides.

Pour the batter into the prepared pan.

Prepare a water bath. I use a shallow pan for water bath as I do not want to create too much moisture in the cake. This is just enough to provide some moisture in the cake so that it’s super soft. I also added a tea towel on the water bath to prevent the pan swimming and moving in the water bath.

Bake it at preheated oven at 150°C/300°F for 30 min, then reduce to 140°C/285°F for 35 min (to prevent cracking on top). Insert a toothpick to make sure the toothpick comes out clean.

Let the cake cool completely before removing from the pan before cutting and serving.