Brew strong espresso (or strong coffee); let cool to room temperature
Separate 4 large eggs; place yolks in a bowl and whites in another bowl
Whisk yolks with 1/2 cup sugar until pale and thick
Whisk in 1 1/4 cups mascarpone until smooth
In a clean bowl, whisk egg whites to stiff peaks
Fold egg whites into the mascarpone mixture until no streaks remain
In a shallow dish, combine cooled espresso with 2–3 tbsp rum or Marsala (optional)
Dip ladyfingers quickly into coffee mixture (do not soak)
Arrange a single layer of dipped ladyfingers in a 9×13-inch dish
Spread half of the mascarpone mixture over the ladyfingers
Add a second layer of dipped ladyfingers
Spread remaining mascarpone mixture on top
Sift cocoa powder generously over the top
Cover and refrigerate at least 4 hours, preferably overnight
Serve chilled