Step 1: Prepare the Mascarpone Cream Base. In a heatproof bowl set over a saucepan of simmering water (bain-marie), whisk the egg yolks with ½ cup of the granulated sugar until the mixture is pale yellow and thickened, about 5-7 minutes. Remove from heat and let cool completely.
Step 2: Whisk in Mascarpone. Once the egg yolk mixture is cool, gently fold in the chilled mascarpone cheese until fully combined and smooth. Be careful not to overmix.
Step 3: Prepare Coffee Mixture. In a shallow dish, combine the cooled espresso or strong coffee with the optional Marsala wine or rum. Stir well.
Step 4: Whip Egg Whites (or Cream). In a separate clean bowl, beat the egg whites with the remaining ¼ cup of granulated sugar until stiff peaks form. Alternatively, if using heavy cream, whip until soft peaks form. Gently fold the whipped whites/cream into the mascarpone mixture in two additions until just combined.
Step 5: Assemble Tiramisu Layers. Quickly dip each ladyfinger into the coffee mixture, one at a time, for just a second or two on each side. Do not over-soak. Arrange a single layer of dipped ladyfingers at the bottom of a 9x13 inch (or similar size) serving dish.
Step 6: Add Cream Layer. Spread half of the mascarpone cream evenly over the layer of ladyfingers.
Step 7: Repeat Layers. Create a second layer of coffee-dipped ladyfingers on top of the cream, then top with the remaining mascarpone cream, spreading it smoothly.
Step 8: Chill and Dust. Cover the dish tightly with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, to allow the tiramisu to set and flavors to meld. Just before serving, dust generously with unsweetened cocoa powder.