Prepare a large 9×13-inch baking dish with a capacity of 12-14 cups and set it aside. You'll need this ready to go for assembling the tiramisu.
Dip the Ladyfingers: Begin by creating two layers of dipped ladyfingers and mascarpone cream. Mix the espresso and Grand Marnier in a shallow bowl. Quickly dip each side of a ladyfinger into this mixture, ensuring it doesn't become too soggy. Arrange the dipped ladyfingers in the baking dish to form a solid layer. If there are gaps, cut some ladyfingers to fit. Save the remaining espresso mixture and ladyfingers for the next layer.
Prepare the Mascarpone Cream: In a large bowl, use a handheld or stand mixer fitted with a paddle or whisk attachment to briefly combine the mascarpone and rum on medium speed. Be careful not to overmix. Set this aside.
Prepare the Egg Yolks: Set up a double boiler or place a heatproof bowl over a pot of simmering water. The water should not touch the bowl. Whisk the egg yolks and 1/4 cup (50g) of granulated sugar together until the mixture is light and foamy, which should take about 5 minutes. Remove from heat and immediately mix with the mascarpone. Beat on medium speed until well combined.
Whip the Heavy Cream: Using a handheld or stand mixer with a whisk attachment, beat the heavy cream and vanilla extract on medium-high speed until medium peaks form, around 3 minutes. Fold this whipped cream into the mascarpone mixture.
Beat the Egg Whites: With a clean whisk attachment, beat the egg whites and salt on medium-high speed until foamy, about 1 minute. Increase to high speed, gradually adding the remaining 1/4 cup (50g) of sugar. Continue beating until stiff peaks form, taking care not to over-beat. Fold these into the mascarpone mixture.
Spread half of the mascarpone cream over the first layer of ladyfingers in an even layer using an offset spatula for neatness.
Dip the Remaining Ladyfingers: Dip the rest of the ladyfingers into the remaining espresso mixture and arrange them on top of the mascarpone layer. Press them down gently to compact. Spread the remaining mascarpone mixture evenly on top, ensuring it fits within the pan even if it towers high. Refrigerate uncovered for 2-3 hours.
Add Cocoa and Chill: After chilling, sift a dense layer of cocoa powder over the top of the tiramisu. Clean the rim of the baking dish, then cover with plastic wrap or aluminum foil. Refrigerate for at least 8-9 more hours, or up to 1 day.
To serve, use a sharp knife to cut the tiramisu into portions, wiping the knife clean between cuts. A small metal spatula can help lift out the slices. Store any leftovers covered in the refrigerator for up to 3 days.