Be sure to use hard, not soft, ladyfingers. Brandy or whiskey can be substituted for the rum. You will need a 3-cup baking dish (measuring approximately 7 ¼ by 5 ¼ inches) or dish of a similar size for this recipe. Do not allow the mascarpone to warm to room temperature before using it or it may curdle.

Stir water, espresso powder, and 1 tablespoon rum together in shallow dish until espresso dissolves; set aside.

Using hand-held mixer set at low speed, beat egg yolks in large bowl until just combined, about 30 seconds. Increase speed to medium-high, add sugar and salt, and beat until pale yellow, 1 to 2 minutes, scraping down sides of bowl as needed. Add remaining 1 tablespoon rum and beat until combined, 15 to 30 seconds. Reduce speed to low, add mascarpone, and beat until no lumps remain, 15 to 30 seconds.

Using hand-held mixer set at medium-low speed, beat cream in small bowl until frothy, about 1 minute. Increase speed to high and beat until stiff peaks form, 1 to 3 minutes. Using rubber spatula, stir one-third of whipped cream into mascarpone mixture, then add remaining whipped cream and gently fold into mascarpone mixture until no white streaks remain.

Working with one at a time, drop half of ladyfingers into espresso mixture, roll to coat, remove, and transfer to 7¼ by 5¼-inch baking dish. (Do not submerge ladyfingers in espresso mixture; entire process should take no longer than 2 to 3 seconds for each cookie.) Arrange soaked cookies in single layer in dish, breaking or trimming ladyfingers as necessary to fit.

Spread half of mascarpone mixture over ladyfingers with spatula, spreading mixture to sides and into corners of dish, then smooth surface. Place 1½ teaspoons cocoa in fine-mesh strainer and dust cocoa over mascarpone mixture.

Repeat dipping and arrangement with remaining ladyfingers; spread remaining mascarpone mixture over ladyfingers and dust with remaining 1½ teaspoons cocoa. Wipe edges of dish clean with paper towel. Cover with plastic wrap and refrigerate for at least 6 hours or up to 24 hours. Sprinkle with grated chocolate, if using, and serve chilled. (Tiramisu can be refrigerated for up to 1 day.)