Step 1: In a medium heatproof bowl, whisk the egg yolks with 1/2 cup (100g) granulated sugar until pale yellow and slightly thickened. Place the bowl over a saucepan of simmering water (make sure the bottom of the bowl does not touch the water). Whisk constantly for 5-7 minutes, until the mixture is hot to the touch and registers 160°F (71°C) on an instant-read thermometer. This pasteurizes the yolks. Remove from heat and let cool slightly.
Step 2: In a separate large bowl, gently fold the room temperature mascarpone cheese with half of the lemon zest. Be careful not to overmix, as mascarpone can curdle. Once the egg yolk mixture has cooled slightly, gradually fold it into the mascarpone until just combined and smooth.
Step 3: In a shallow dish, combine the limoncello liqueur, 1/2 cup (120ml) cold water, and 1 tbsp granulated sugar (if using). Stir until the sugar dissolves.
Step 4: Quickly dip each ladyfinger into the limoncello syrup, ensuring it's moistened but not soggy. A quick 1-2 second dip per side is usually sufficient. Arrange a single layer of dipped ladyfingers in the bottom of an 8x8 inch (20x20 cm) baking dish or a similarly sized trifle bowl, breaking them as needed to fit.
Step 5: Spread half of the mascarpone cream evenly over the first layer of ladyfingers.
Step 6: Place a second layer of dipped ladyfingers on top of the cream.
Step 7: Cover with the remaining mascarpone cream, smoothing the top with an offset spatula.
Step 8: Cover the dish tightly with plastic wrap and refrigerate for at least 4-6 hours, or preferably overnight, to allow the flavors to meld and the tiramisu to set.
Step 9: Just before serving, dust generously with powdered sugar and/or the remaining fresh lemon zest. Slice and serve cold.