Traditional Italian Tiramisu

2½ c. prepared espresso, room temperature
9 Tbsp. dark rum
6 large egg yolks
⅔ c. sugar
¼ tsp. table salt
1½ lb. mascarpone cheese
¾ c. cold heavy whipping cream
14 oz. ladyfingers
3½ Tbsp. natural cocoa
¼ c. semisweet or bittersweet chocolate, grated

Stir the espresso and 5 tablespoons of the rum in a wide bowl or baking dish; set aside.
In the bowl of a stand mixer fitted with the whisk attachment, beat the egg yolks at low speed until just combined. Add the sugar and salt and beat at medium-high speed until pale yellow, 1½ to 2 minutes, scraping down bowl with rubber spatula once or twice. Add the remaining 4 tablespoons of rum and beat at medium speed until just combined, 20 to 30 seconds; scrape bowl. Add the mascarpone and beat at medium speed until no lumps remain, 30 to 45 seconds, scraping down bowl once or twice. Transfer mixture to large bowl and set in the refrigerator to keep cool.
In the now-empty mixer bowl (no need to clean bowl), beat the cream at medium speed until frothy, 1 to 1½ minutes. Increase speed to high and continue to beat until cream holds stiff peaks, 1 to 1½ minutes longer. Using a rubber spatula, fold one-third of the whipped cream into the mascarpone mixture to lighten, then gently fold in remaining whipped cream until no white streaks remain. Set mascarpone mixture in the refrigerator again to keep cool.
Working one at a time, drop half of the ladyfingers into the coffee mixture, roll, remove, and transfer to 13 by 9-inch glass or ceramic baking dish. (Do not submerge ladyfingers in coffee mixture; entire process should take no longer than 2 to 3 seconds for each cookie.) Arrange soaked cookies in single layer in baking dish, breaking or trimming ladyfingers as needed to fit neatly into dish.
Spread half of the mascarpone mixture over ladyfingers; use rubber spatula to spread mixture to sides and into corners of dish and smooth surface. Place 2 tablespoons of cocoa in a fine-mesh strainer and dust cocoa over mascarpone.
Repeat the dipping and arrangement of ladyfingers; spread remaining mascarpone mixture over ladyfingers and dust with remaining 1½ tablespoons of cocoa. Wipe the edges of the dish with a dry paper towel, cover with plastic wrap, and refrigerate 6 to 24 hours. Sprinkle with grated chocolate, if using; cut into pieces and serve chilled.