Brandy and even whiskey can stand in for the dark rum. The test kitchen prefers a tiramisù with a pronounced rum flavor; for a less potent rum flavor, halve the amount of rum added to the coffee mixture in step 1. Do not allow the mascarpone to warm to room temperature before using it; it has a tendency to break if allowed to do so. Be certain to use hard, not soft ladyfingers. Cooking time is chill time.