Preheat your oven to 375 F degrees. Spray a 9x13 inch baking pan with cooking spray. Set aside.
Separate the eggs, placing the egg whites in a bowl for later. In another bowl add the egg yolks, the 2/3 cup of sugar, vanilla extract and beat with a mixer on high speed until the mixture is thick and pale. Should take about 2 minutes.
Into another bowl whisk together the flour, salt and baking powder. Add about half the flour mixture to the egg yolk mixture and whisk until well combined. Whisk in the milk, then add the remaining flour mixture and whisk well. The cake batter will be quite thick.
Beat the egg whites with a mixer on high speed until stiff peaks form. Add the 3 tbsp sugar and continue beating for another minute until sugar is well incorporated. Gently fold the egg whites into the cake batter.
Pour the batter into the prepared baking pan. Bake for 20 minutes or until a toothpick inserted in the center of the cake comes out clean. Cool the cake for 10 minutes.
While the cake is cooling, prepare the milk sauce by whisking all the milk sauce ingredients into a bowl. Use as much coffee as you like, more for a stronger coffee flavor. I used Nescafe Espresso.
Pierce the entire of the the cake with a fork. Slowly pour the milk mixture over the entire cake, evenly. Cover the pan with plastic wrap and refrigerate for at least 3 hours to overnight.
In another bowl prepare the topping by whisking together the mascarpone cheese with the powdered sugar until smooth. Add the cool whip or whipped cream and continue whisking until fully incorporated and smooth.
Top the cake with the mascarpone topping and if you'd like sprinkle some shaved chocolate over the top. You can also dust it with cocoa powder instead and/or serve it up with strawberries or raspberries.