Preheat the oven to 325° Fahrenheit. Grease a 9x9 inch square pan with butter or cooking spray, set aside. Separate the egg whites and yolks into separate large bowls (be sure that NO yolk gets into the egg whites). Add the vinegar into the egg whites and, using an electric mixer, beat the whites until stiff peaks form (a few minutes at least), then set aside.

In the egg yolk bowl, add in the sugar and water and beat, using the same mixer, until lighter in color and fluffier. Slowly add in the melted butter, beating on low speed until combined. Do the same thing with the flour and cocoa powder (add slowly as you beat slowly). Finally, add in the salt and vanilla extract and beat until smooth and combined. Then, using a spatula, slowly stir in the two cups of warm milk until fully mixed. The batter will be very liquid at this point. Then, using the same spatula, gently fold in 1/3 of the egg whites at a time until all the lumps of egg white fluff are smoothly mixed into the chocolate batter (this may take a few minutes but do not quickly stir it to get rid of the lumps, take your time). The batter should be very liquidy.

Pour the batter into the greased pan and bake for 45-50 minutes until the center doesn't jiggle (do not over-bake). Let the cake cool completely before serving! The fluffy egg whites will rise to the top to form a light chocolate sponge, and the rest will settle at the bottom for the chocolate pudding layer.