First, start by separating your egg whites from the egg yolks.
Next, in a stand mixer or mixing bowl using a hand mixer, cream together the softened unsalted butter, white granulated sugar and egg yolks until it has a very creamy consistency.
Add in the vanilla extract and continue mixing.
In a separate bowl, add the flour, unsweetened cocoa powder, baking powder and salt. Whisk together to break up any lumps from the flour and unsweetened cocoa powder.
Add the flour and cocoa powder mixture and the milk to the butter, sugar and egg yolk mixture. The flour and cocoa powder mixture should be added 1/4 cup at a time while mixing to avoid a cloud of flour dust from messing up your kitchen. Alternate adding the milk little by little as well and continue stirring throughout this process. Stop mixing to scrape the sides of the bowl as needed.
Add the chocolate chips to the cake batter and fold them in using a spatula.
Preheat the oven to 350 degrees F.
Transfer the cake batter to a separate bowl because you will need a clean stand mixer or bowl to mix the egg whites. If you were using the stand mixer, thoroughly clean that bowl out and dry it before adding the egg whites.
Add the egg whites to a clean bowl or stand mixer and begin mixing until stiff peaks form.
Gently fold the egg whites into the cake batter you prepared earlier until all of it is well incorporated. Do not over mix.
Grease a 9×13 baking dish.
Pour the cake batter into the baking dish and bake right away in a preheated 350 degree F oven for 20-25 minutes or until a toothpick inserted in the center comes out clean.
Remove the chocolate cake from the oven and allow it to cool. Use a fork to poke holes all over the cake.