Prepare a large 9x13-inch baking pan and set it aside. Line the pan with parchment if you prefer easier serving.

In a shallow bowl, whisk together the very strongly prepared espresso and the Grand Marnier if using.

Quickly dip each ladyfinger into the espresso mixture, one at a time, then arrange them in a single layer in the baking pan.

Tip: dip just long enough to moisten the ladyfinger without letting it fall apart.

In a mixing bowl, combine the cold mascarpone and the 2 tablespoons dark rum. Stir or beat gently until smooth and lump-free.

Set a double boiler: place a heatproof bowl over gently simmering water. Whisk the egg yolks with 1/4 cup granulated sugar in the double boiler until the mixture is foamy and slightly thickened.

Fold the warmed egg yolk mixture into the mascarpone and rum mixture until fully incorporated and smooth.

In a separate chilled bowl, whip the cold heavy cream with the 1/2 teaspoon pure vanilla extract and 1/8 teaspoon salt until medium peaks form.

Gently fold the whipped cream into the mascarpone mixture until combined.

In another clean bowl, beat the egg whites with the remaining 1/4 cup granulated sugar until stiff peaks form.

Carefully fold the beaten egg whites into the mascarpone mixture in two additions.

Spread half of the mascarpone cream evenly over the layer of soaked ladyfingers in the pan.

Dip the remaining ladyfingers quickly into the espresso mixture and arrange them in a single layer on top of the cream layer.

Spread the remaining mascarpone cream over the second layer of ladyfingers.

Refrigerate the tiramisu uncovered for 2 to 3 hours to set the layers.

Dust the top with cocoa powder after chilling.

Cover the pan and refrigerate for at least 8 to 9 hours or overnight for best flavor and texture.

Slice and serve chilled. For cleaner slices, run a warm knife under hot water, dry it, and then slice.