- 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.)
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