Step 1: In a heatproof bowl, combine egg yolks and granulated sugar. Place the bowl over a saucepan of simmering water (ensure the bowl doesn’t touch the water). Whisk continuously until the mixture is pale, thick, and has increased in volume, about 5-7 minutes. Remove from heat and let cool slightly.
Step 2: In a large bowl, gently whisk the room-temperature mascarpone cheese until smooth. Fold the slightly cooled egg yolk mixture into the mascarpone until just combined. Be careful not to overmix.
Step 3: In a separate very clean, dry bowl, whip the egg whites until stiff peaks form. Gently fold the whipped egg whites into the mascarpone mixture in two additions, using a rubber spatula, until fully incorporated and airy.
Step 4: Pour the cooled espresso (and liqueur, if using) into a shallow dish. Quickly dip each ladyfinger into the coffee mixture, about 1-2 seconds per side. Do not oversoak. Arrange a single layer of dipped ladyfingers at the bottom of a 9×13 inch baking dish.
Step 5: Spread half of the mascarpone cream evenly over the first layer of ladyfingers, smoothing with a spatula.
Step 6: Repeat with another layer of coffee-dipped ladyfingers, followed by the remaining mascarpone cream. Smooth the top surface.
Step 7: Cover the dish tightly with plastic wrap and refrigerate for at least 4-6 hours, or preferably overnight, to allow the tiramisu to set and flavors to meld. Just before serving, dust generously with unsweetened cocoa powder using a fine-mesh sieve.