In a clean bowl, whip the egg whites and 3 tbsp of sweetener together with a hand mixer, for about 3-5 minutes until the egg whites hold stiff peaks.
In a separate bowl, whip the egg yolks with the remaining 3 tbsp sweetener for 2-3 minutes until the egg yolks are thick and pale yellow in color.
Add the mascarpone to the egg yolks and whip until combined.
Gently fold the stiff egg whites into the egg yolk mixture and set aside.
In a small flat dish or bowl, combine the espresso and rum extract. Dunk each ladyfinger into the espresso mixture for a few seconds. These homemade ladyfingers are more dense than their store bought counterparts, so you can let them soak for 5-10 seconds per side.
Place soaked cookies into the bottom of a 8x8 dish.
Once the ladyfingers have formed a single layer in the bottom of the dish, spread 1/2 of the custard mixture over the ladyfingers.
Arrange another layer of espresso soaked ladyfingers on top, and spread over the remaining custard. Dust top with cocoa powder if desired.
Cover the top of the dish with plastic wrap and let the tiramisu refrigerate for 4-6 hours. It does need to sit this long before serving.
Serve cold. Enjoy!