Make Zabaglione:
Prepare bain-marie: Fill saucepan with 2 inches water, bring to simmer. Place heatproof bowl on top (bottom not touching water).
Whisk 6 egg yolks + 3/4 cup granulated sugar in bowl until pale and thick (about 5 minutes).
Gradually add ? cup Marsala wine (or dark rum), whisking constantly. Cook 8-10 minutes until mixture triples in volume, reaches 71°C/160°F, and leaves ribbons.
Remove from heat, continue whisking 2 minutes to cool slightly.
Prepare Cream Base:
In separate bowl, beat 500g mascarpone (room temperature) with spatula until smooth.
Fold mascarpone into cooled zabaglione in 3 additions until fully incorporated.
In chilled bowl, whip 300ml heavy cream (35% fat) to medium peaks.
Gently fold whipped cream into mascarpone mixture until uniform.
Assemble:
Brew 2 cups strong espresso, cool to room temperature. Stir in 2 tbsp Marsala or rum.
Quickly dip 24 Savoiardi ladyfingers (one at a time) into coffee for 1 second per side — moist but not soggy.
Arrange 12 dipped ladyfingers in single layer in 9×13 inch dish.
Spread half cream mixture (about 3 cups) evenly over ladyfingers.
Repeat with remaining ladyfingers and cream.
Cover with plastic wrap touching surface. Refrigerate 6 hours minimum, preferably overnight.
Serve:
Just before serving, dust entire surface generously with unsweetened cocoa powder using fine-mesh sieve.
Use warm knife (dipped in hot water, wiped dry) for clean cuts.
Serve within 2 minutes of dusting to prevent cocoa absorption.