Separate your eggs carefully, ensuring no yolk gets into the whites—even a tiny amount of yolk will prevent the whites from whipping properly. In a large bowl, whisk the egg yolks with half the sugar (1/4 cup) for 5-7 minutes until the mixture becomes pale, thick, and ribbony; this aerating process is crucial for a light, fluffy texture. Add the room-temperature mascarpone cheese and fold gently until just combined—don't overmix or the mascarpone will become grainy. Set this mixture aside. I like to use turbinado sugar here because it creates a slightly grainier, more interesting texture than regular granulated sugar.

In a separate, completely clean bowl (any residual fat will prevent whipping), beat the egg whites with the remaining 1/4 cup sugar for 5-7 minutes until stiff peaks form. For food safety with raw eggs, I gently heat the egg whites in a double boiler over simmering water to 140°F, whisking constantly for about 1-2 minutes, then remove from heat and continue beating until stiff peaks form—this pasteurizes them without cooking them solid. Gently fold the whipped egg whites into the mascarpone mixture from Step 1 in two additions. Once partially combined, add the freshly grated nutmeg and fold until fully incorporated with no white streaks remaining.

While the cream chills slightly, combine the cooled espresso with the rum (or brandy, which I prefer for a deeper, more sophisticated flavor) in a shallow dish. Stir well to ensure the alcohol is evenly distributed. For this step to work smoothly, your espresso must be completely cooled—if it's warm, it will make the ladyfingers soggy too quickly.

Working quickly, dip each ladyfinger into the espresso mixture for just 1-2 seconds per side—they should be moistened but not soaked, as they'll absorb more liquid as the dessert sits. Arrange a single layer of dipped ladyfingers in your serving dish (a 9x13-inch dish works well, or individual glasses if you prefer). Spread half of the eggnog mascarpone cream from Step 2 evenly over the ladyfingers. Repeat with another layer of dipped ladyfingers followed by the remaining cream. Dust the top generously with a pinch of additional nutmeg for color and aroma.

Cover the assembled tiramisu and refrigerate for at least 4 hours, or preferably overnight—this resting period allows the flavors to meld and the texture to set properly. The mascarpone cream will firm up slightly and the espresso flavors will deepen. Serve directly from the refrigerator; tiramisu is best enjoyed cold and should be kept chilled until service.