The coffee soak is essential for authentic tiramisu character, transforming plain sponge into something moist and flavor-saturated. Apply generously but not to the point of sogginess; the sponge should be noticeably moist without becoming structural weak or falling apart. Dissolve sugar in warm coffee before cooling to ensure even sweetness distribution. Coffee liqueur adds traditional boozy depth but can be omitted for a family-friendly version. Apply soak with a pastry brush for even coverage, allowing each layer to absorb before adding filling. The cake will continue absorbing soak as it sits, so assembled tiramisu improves over 24 hours in the refrigerator.