Prep: Preheat oven to 325°F. Line a 8x8 inch baking dish with parchment paper.
Mix the yolks with water and sugar: Separate the eggs and add the egg yolks to the bowl of a mixer. Add a tbsp of water and the sugar. Beat this at high speed until are eggs are light and pale in color.
Drizzle in melted butter, vanilla and add salt: Add melted butter, vanilla extract and salt to the yolk mixture and continue beating until butter is fully incorporated and the mixture is light.
Stir in the dry ingredients: Add the flour and cocoa powder to the bowl and mix thoroughly, scraping down the side and bottom of the bowl as needed.
Add milk: 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 wet, so don't get scared.
Whisk the egg whites: If using a handheld mixer, clean the beaters and pour the egg whites in a large clean bowl. If using a stand mixer, wash the mixing bowl and whisk attachment and make sure you dry them out. Add egg whites to mixing bowl and the vinegar and mix until egg whites are stiff.
Add the egg whites to cake batter: Add a bit of the egg whites to the chocolate mixture and gently mix using a whisk. 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 mixed together and the mixture should be quite thin. Make sure to leave a distinct layer of fluffy egg white lumps at the top of the batter, so do not overmix.
Bake, chill and serve: 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, chill for about 4 hours letting the cake set, then cut into squares and serve.