Mix the egg yolks with 2 ounces sugar, and mix until you obtain a creamy light mixture. Work the mascarpone in a bowl using a wooden spoon, making sure you eliminate any lumps, then add the mascarpone to the sugar-egg mixture and continue to mix well.
In a separate bowl, mix the egg whites, pinch of salt, and the remaining 2 ounces sugar, and beat them up with a mixer until they reach a somewhat firm, but fluffy consistency, then add them to the mascarpone mixture. Stir in the rum, if using.
Dip the savoiardi cookies (firm ladyfingers) in the coffee, and one by one lay them flat into a 7 by 11 pyrex tray, making sure you do not soak the cookies, as you want to make sure they maintain their firmness. Once the first layer of cookies has been laid out, spread a layer of the mascarpone cream on top, and dust with 1 tablespoon cocoa powder.
Now, again, prepare another layer of coffee-dipped cookies, cream and cocoa powder. Garnish the top of the cake with the shaved dark chocolate.
Cover the tray with plastic wrap and refrigerate for about 3 hours so the flavors can marry and the tiramisu can settle.