In a large bowl, add in the egg yolks, milk, and oil. Mix together with a whisk.
Sift in the flour and cocoa powder. Whisk until well combined then set this bowl aside.
In a separate large bowl, add in the egg whites and cream of tartar. Beat using a hand mixer until it turns frothy then slowly add in the sugar 1/3 at a time. Beat until you get stiff peaks.
Add some of the egg white mixture to the egg yolk mixture. Gently mix together then pour this mixture into the remaining egg white mixture. Gently fold until everything is well combined.
Transfer cake batter to cake pan
Run a skewer through the batter to remove any air bubbles. Also drop the cake pan a few times to remove air bubbles.
Bake the cake at 135C/275F for 50 minutes
Let the cake cool upside down before removing from the pan.