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1
Stir coffee, espresso, and 5 tablespoons rum in wide bowl or baking dish until espresso dissolves; set aside.
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2
In bowl of standing mixer fitted with whisk attachment, beat yolks at low speed until just combined. Add sugar and salt and beat at medium-high speed until pale yellow, 1 1/2 to 2 minutes, scraping down bowl with rubber spatula once or twice. Add remaining 4 tablespoons 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. Transfer mixture to large bowl and set aside.
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3
In now-empty mixer bowl (no need to clean bowl), beat cream at medium speed until frothy, 1 to 1 1/2 minutes. Increase speed to high and continue to beat until cream holds stiff peaks, 1 to 1 1/2 minutes longer. Using rubber spatula, fold one-third of whipped cream into mascarpone mixture to lighten, then gently fold in remaining whipped cream until no white streaks remain. Set mascarpone mixture aside.
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4
Working one at a time, drop half of ladyfingers into 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.
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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 tablespoons cocoa in fine-mesh strainer and dust cocoa over mascarpone.
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6
Repeat dipping and arrangement of ladyfingers; spread remaining mascarpone mixture over ladyfingers and dust with remaining 1 1/2 tablespoons 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.
