Stir 1 ½ cups strongly brewed coffee, 1 tablespoon granulated sugar, 2 teaspoons dark rum if using, and ½ teaspoon vanilla extract together in a medium bowl until the sugar is dissolved.

Beat ½ cup cold heavy cream and 2 tablespoons of the granulated sugar together in a medium bowl with an electric hand mixer on medium speed until stiff peaks form, 2 to 3 minutes.

Beat 5 large egg yolks and the remaining ½ cup granulated sugar together in a large bowl with the mixer on medium speed until the mixture is pale yellow and glossy, about 3 minutes. Add 16 ounces cold mascarpone cheese and gently fold together with a flexible spatula until almost combined (a few streaks are okay).

Add the whipped cream and fold just until no streaks remain.

Working with 1 ladyfinger at a time, quickly dip and submerge in the coffee soak, then place in an 8x8-inch baking pan. Repeat, leaving a little space in between each one and breaking ladyfingers as needed to fit, until the bottom of the baking dish is covered, about 9 ladyfingers.

Transfer half of the mascarpone filling (about 2 cups) over the ladyfingers and spread into an even layer. Repeat with a second layer of soaked ladyfingers, then the remaining mascarpone filling. Sift 1 tablespoon unsweetened cocoa powder evenly over the top.

Refrigerate for at least 4 hours but preferably overnight. Shave enough of a 1 (4-ounce) bar dark chocolate with a vegetable peeler over the top to cover if desired before serving.