Stir together coffee or espresso, 1 Tbsp sugar and dark rum in a shallow bowl then set aside to cool.
Heat a small amount of water to simmer in a medium saucepan. Whisk egg yolks and ½ cup granulated sugar together in metal bowl or glass heatproof bowl and position over the saucepan of barely simmering water (simmering water should not touch the bottom of the bowl). Whisk or use a hand mixer to continuously beat the egg yolk mixture as it heats (4−5 minutes), until the mixture has tripled in volume and becomes pale yellow in color. Remove the egg yolk mixture from heat, add mascarpone then stir until combined and no lumps remain. If using whipped cream, fold gently into mascarpone mixture.
Collect components for assembling tiramisu (espresso, ladyfingers, mascarpone mixture, cocoa powder and an 8" square baking dish).
Begin by dipping each ladyfinger in espresso mixture, dunking several times to allow time for coffee to absorb (about 3 seconds)—cookie should be wet and soft, but not soggy and saturated. Immediately transfer the cookie to the baking dish (work quickly, as the ladyfinger will become very soft when the coffee soaks in) and arrange in bottom of baking dish. You'll need 12 coffee-soaked ladyfingers to cover the bottom of the baking dish (2 rows of 6 cookies each).
Spread half of the mascarpone mixture evenly over bottom layer of ladyfingers. Repeat soaking procedure with remaining 12 ladyfingers, arranging them on top of the mascarpone layer. Spread the remaining mascarpone mixture over the second layer of ladyfingers.
Sprinkle the top of your tiramisu with cocoa or shaved chocolate. Cover and chill at least 2 hours and then serve and enjoy!
One Last Thing
It only takes 20 minutes to put this dish together, but it needs a good 2 hours in the fridge so the flavors have a chance to meld together!