1. Stir coffee, espresso, and 1 Tbsp rum in wide bowl or baking dish until espresso dissolves; set aside.

2. In large bowl, beat yolks at low speed until just combined. Add sugar and salt and beat at medium-high speed until pale yellow, 1.5 to 2 minutes, scraping down bowl with rubber spatula once or twice. Add remaining 4 tsp rum and beat at medium speed until just combined 20 to 30 seconds; scrape bowl. Add mascarpone and beat at medium speed until no lumps remain, 30 to 45 seconds, scraping down bowl once or twice.

3. In another bowl, beat cream at medium speed until frothy, 1 to 1.5 minutes. Increase speed to high and continue to beat until cream holds stiff peaks, 1 to 1.5 minutes longer. Using rubber spatula, fold one-third of whipped cream into mascarpone mixture to lighten, and then gently fold in remaining whipped cream until no white streaks remain. Set mascarpone mixture aside.

4. Working one at a time, drop half of ladyfingers into coffee mixture, roll, remove, and transfer to a 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 dipped cookies in single layer in baking dish, breaking or trimming ladyfingers as needed to fit neatly into dish.

5. Spread half of mascarpone mixture over ladyfingers; use rubber spatula to spread mixture to sides and into corners of dish and smooth surface. Place 2 tsp cocoa in fine-mesh strainer and dust cocoa over mascarpone mixture.

6. Repeat dipping and arrangement of ladyfingers; spread remaining mascarpone mixture over ladyfingers and dust with remaining cocoa. Wipe edges of dish with 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.