Separate the eggs carefully. Place the egg yolks in a medium heat-safe bowl and add 6 tablespoons (75g) of the sugar. Whisk vigorously until pale yellow, thick, and ribbon-like, about 2-3 minutes. Set aside.
Add the cold mascarpone to the egg yolk mixture. Whisk gently until smooth and no lumps remain, about 30 seconds. Avoid over-mixing. Set aside.
In a clean, dry large bowl, beat the egg whites on medium speed until foamy. Gradually add the remaining 6 tablespoons (75g) sugar and whip on medium-high until stiff, glossy peaks form, 3-5 minutes. Do not over-beat.
Gently fold one-third of the whipped egg whites into the mascarpone mixture to lighten it. Fold in the remaining whites in two additions until no streaks remain. The cream should be light and airy.
Pour the cooled espresso into a shallow dish. Stir in the coffee liqueur or Marsala if using. Taste and adjust sweetness if needed.
Quickly dip each ladyfinger in the coffee mixture for 1-2 seconds. Arrange in a single layer in the baking dish. Spread half the mascarpone cream evenly over the top.
Repeat dipping and layering with remaining ladyfingers for a second layer. Spread the remaining mascarpone cream on top and smooth evenly.
Cover tightly with plastic wrap and refrigerate for at least 4 hours, preferably overnight. Just before serving, dust generously with cocoa powder using a fine-mesh sieve.