Prepare Your Baking Dish: You’ll need a 9×13-inch pan or dish that holds at least 12-14 cups. Set it aside so it’s ready for assembly.
Dipping the First Layer of Ladyfingers: Combine the espresso and Grand Marnier (if using) in a shallow bowl. For the first layer, quickly dip each side of a ladyfinger into the espresso mixture, ensuring not to soak them too much as they’ll become mushy. Place each dipped ladyfinger into the baking pan, forming a solid layer. If necessary, trim the ladyfingers to fill any gaps. Reserve the rest of the espresso mixture and ladyfingers for the next layer.
Make the Mascarpone Mixture: In a large mixing bowl, beat the mascarpone cheese and dark rum together with a handheld or stand mixer (using either the paddle or whisk attachment) on medium speed until just combined. Be cautious not to overmix; set aside.
Prepare the Egg Yolks: If you have a double boiler, use it here. If not, place a heatproof bowl over a pot of simmering water, ensuring the water does not touch the bowl's bottom. Whisk the egg yolks with 1/4 cup (50g) of sugar until the mixture is light and foamy, about 5 minutes. Immediately add this to the mascarpone mixture and beat on medium speed until well blended.
Whip the Cream: In another bowl, whip the heavy cream with the vanilla extract using a mixer until it reaches medium peaks, which should take around 3 minutes. Medium peaks hold their shape but are not overly stiff. Gently fold the whipped cream into the mascarpone mixture until well combined.
Beat the Egg Whites: Clean the whisk attachment and, in a separate bowl, beat the egg whites with salt on medium-high speed until foamy, about 1 minute. Gradually increase the speed to high, adding the remaining 1/4 cup (50g) of sugar slowly. Continue beating until stiff peaks form, approximately 4-5 minutes. Avoid over-beating as the egg whites may become dry. Carefully fold these into the mascarpone cream.
Assemble the Layers: Spread half of the mascarpone cream over the first layer of ladyfingers in the pan, using an offset spatula for a smooth finish.
Dip the Second Layer of Ladyfingers: Dip the remaining ladyfingers in the espresso mixture and arrange them over the mascarpone layer. Lightly press each ladyfinger down to compact them. Spread the rest of the mascarpone mixture on top, ensuring an even and neat layer.
Chill Before Adding Cocoa: Refrigerate the assembled tiramisu uncovered for 2-3 hours. After this initial chilling period, dust a generous layer of cocoa powder over the top using a fine-mesh sieve. Clean the edges of the baking dish, cover with plastic wrap or foil, and refrigerate for an additional 8-9 hours or up to 24 hours for the best texture.
Serving: To serve, cut the tiramisu into squares using a sharp knife, wiping the blade clean between slices. Use a small metal spatula to lift each slice for easy serving.
Storage: Store any leftovers in the refrigerator, covered, for up to 3 days to maintain freshness.