Preheat your oven to 375 F.
Separate your egg yolks from whites.
Add sugar and salt to your egg yolks and mix thoroughly.
Start by adding small quantities of flour and butter and mix. This process will avoid you ending up with any lumps in your batter. You should end up with a very thick batter.
Now beat your egg whites until you form a meringue consistency. This is like a shaving cream consistency. Check picture above. If you are doing this process by hand, use a large bowl and do long strokes. FYI : Great arms workout.
Now add your meringue to the batter and whisk with a spatula. Once you have a homogenous batter you will notice that it is now much lighter.
Split your batter between two bowls. Leave one bowl aside and to the other bowl add the cocoa powder and mix. Now add in your chocolate chips and fold them into the mixture.