Method

• 1.
  Line a 4 × 8-inch loaf pan with two crisscrossed pieces of plastic wrap, pressing them into the corners and allowing a few inches to hang over the sides (to get the plastic to stick, wet the pan lightly), then set aside.
• 2.
  In a shallow bowl, combine the brewed coffee and Kahlua and set aside.
• 3.
  Place the yolks in a medium bowl and set aside. Have a hand mixer at the ready. In a small saucepan, combine the sugar, instant coffee, vanilla, salt, and 1/4 cup (2 oz / 57g) water and bring to a boil over medium heat, stirring with a heatproof flexible spatula to dissolve the sugar and coffee granules. When the mixture comes to a boil, remove the saucepan from the heat and slowly stream the syrup into the yolks, beating constantly with the hand mixer on medium speed (the hot syrup will pasteurise the yolks; add it slowly so you don’t scramble them). Once you’ve added all the syrup, increase the speed to medium-high and continue to beat until the yolk mixture is very light, thick, and mousse-y and has cooled to room temperature, about 3 minutes. Set the bowl aside.
• 4.
  In a large bowl, combine the mascarpone and 1/2 cup (4.2 oz / 120g) of the heavy cream and beat with the hand mixer (no need to wash it first) on low speed to start and gradually increase the speed to medium-high as it thickens until the mixture holds a stiff peak, about 1 minute. Stop the mixer and scrape in the yolk mixture. Mix on low to combine, then increase the speed to medium-high and beat until it holds stiff peaks again, 1 to 2 minutes longer. Set the bowl aside.
• 5.
  Trim the ends off 6 or 7 ladyfingers (they need to fit crosswise along the bottom of the loaf pan in a tight-fitting row) and set the ladyfingers aside. Scrape a level 1/2 cup (2.5 oz / 70g) of the filling into the lined loaf pan and use the back of a spoon to spread it across the bottom all the way to the sides and corners in an even layer. Quickly dunk the trimmed ladyfingers, one at a time, into the coffee-Kahlua mixture, then place them crosswise in a row in the pan. Scrape a level 1 cup (4.9 oz / 140g) of the filling over the ladyfingers in the pan, spreading it in an even layer. Repeat the trimming, dipping, and arranging process for a second layer of ladyfingers and top with another 1 cup of the filling, spreading it evenly. Add a third crosswise layer of dipped ladyfingers, noting that you won’t have to trim them this time because the top of the pan is wider than the base. Scrape the remaining filling on top and smooth it all the way to the sides and into the corners.
• 6.
  Wrap the plastic up and over the top of the pan and press it gently onto the surface of the filling. Refrigerate until the filling is set and the ladyfingers are completely softened, at least 12 hours.
• 7.
  Just before serving, in a medium bowl, with a whisk or the hand mixer, whip the remaining 1 cup (8.5 oz / 240g) cream on medium-low speed to start and gradually increase the speed to medium-high as it thickens, until you have medium peaks.
• 8.
  Remove the loaf pan from the refrigerator and peel back the plastic from the surface. Tug gently on the overhang of the plastic to help loosen the icebox cake from the pan. Place a plate or serving platter upside down over the pan, making sure the ends of the plastic are out of the way, then invert the plate and pan together. Remove the pan, pulling downward on the plastic to help release it if needed, then carefully peel away the plastic. Scrape the whipped cream over the icebox cake and spread it across the top and down the sides. Sprinkle the icebox cake lightly with cocoa powder, then cut crosswise into slices and serve.