Preheat oven to 350°F and line a baking sheet with parchment paper.
In a medium bowl, beat the egg whites and cream of tartar until you get soft peaks. Add 2 tablespoons of the sugar to the egg whites and beat until you get firm peaks. Place the bowl in refrigerator to keep the mixture chilled.
In a separate medium bowl, beat the egg yolks, remaining sugar and vanilla extract until smooth.
In a small bowl, whisk the flour and baking powder together.
Gently fold half of the egg whites into the egg yolk mixture and then half of the flour mixture. Fold remaining egg whites and then remaining flour mixture until fully incorporated.
Fill a pastry bag with the batter and pipe the batter onto the parchment paper in 3 inch lengths.
Bake for 7-8 minutes. (Do not over-bake, lady fingers will be soft and moist.)Set aside and let cool while preparing the mascarpone filling.
In a cold mixing bowl, beat the heavy whipping cream until stiff peaks form and place in refrigerator to chill.
Combine the egg yolks and sugar in a glass bowl and whisk over a pot of lightly boiling water for 12 minutes until thick.(Use a double boiler if you have one for this step.)
Take the bowl off of the pot of water and place it on top of a bowl of ice to cool the mixture down.
Combine the egg yolk mixture with the mascarpone cheese in a mixing bowl and beat until smooth. Fold the whipped cream into the mascarpone cheese mixture until fully incorporated.
Line the bottom of an 8" x 8" baking dish with a layer of lady fingers. Using a pastry brush, brush coffee liquor onto the lady fingers. (If you buy pre-made lady fingers that are hard, you may have to dip them in strong coffee to softened them before you brush the coffee liquor onto the biscuits.)
Spread a third of of the mascarpone filling on top of the lady fingers and then dust with the cocoa powder. Continue with two more layers each, the last thing being the dusting of cocoa powder on top.
Chill for a minimum of 4 hours or overnight. Serve chilled.
Store in the fridge, loosely covered, for up to 3 days.