Preheat oven to 325 F degrees. Grease and flour an 8x8 inch baking pan.
Separate the eggs and add the egg yolks to the bowl of a mixer. Add a tbsp of water to the egg yolks and add the sugar. Beat this at high until eggs are light and creamy.
Add melted butter, vanilla extract and salt to mixer and continue beating until butter is fully incorporated and mixture is light and fluffy.
You can mix the cocoa and flour together and add to the mixing bowl in batches, 3 times should be enough. After each addition mix thoroughly.
Once flour and cocoa powder has been mixed in, add the milk a bit at a time and mix. The milk should be warm so not to cool the butter. Make sure the milk has been mixed in thoroughly, the mixture should be pretty liquidy, so don't get scared.
Clean the mixing bowl and whisk attachment and make sure you dry it out. Add egg whites to mixing bowl and the vinegar and mix until egg whites are stiff.
Add a bit of the egg whites to the chocolate mixture and gently fold in. Next you're going to reverse and add a bit of the chocolate mixture to the egg whites. This way it will be easier to mix everything together. Repeat until everything is well mixed and the mixture should be thin like a pancake batter.
Pour the batter in the prepared baking dish and bake for roughly 60 minutes or until done. The cake will still jiggle in the middle when you take it out, so that's normal. Let it cool completely then cut into squares and serve.
Please keep in mind that nutritional information is a rough estimate and can vary greatly based on products used.