Set a large metal bowl over a medium pot of simmering water. In the bowl, combine the egg yolks, ½ cup of the sugar, 2 tablespoons of the Kahlúa, the vanilla, and the salt. Heat, whisking continuously, until the mixture is pale yellow, thickened, and tripled in volume, 6-8 minutes. Remove from the heat and stir in the mascarpone.
In another large bowl, using a hand-held mixer, whip the heavy cream to stiff peaks. Gently fold the whipped cream into the egg mixture.
In a shallow baking dish or quarter sheet pan, stir together the hot coffee, remaining 2 tablespoons sugar, and remaining ¼ cup Kahlúa until the sugar has dissolved.
To assemble, spread ⅓ of the cream on the bottom of a 2-quart casserole dish or 9-inch square baking dish. Dip half of the cookies in the coffee mixture and line up over the cream. Repeat this process with another ⅓ of cream and soaked cookies, followed by the remaining cream. Cover and refrigerate for at least 4 hours.
Using a fine-mesh sieve, dust the unsweetened cocoa over the tiramisu right before scooping and serving.
