Step 1
Lightly moisten an 8½x4½” loaf pan (this will cause the plastic to stick), then cover with 2 crisscross pieces of plastic wrap, pressing into corners and leaving ridges on all sides. Combine ¾ cup strong brewed coffee Y ¼ cup Kahlua in a shallow container and reserve.
Step 2
Place 3 large egg yolks, room temperature, in a medium bowl. Bring ¼ cup granulated sugar, 2 teaspoons instant espresso powder, 1 teaspoon vanilla extract, ½ tsp. Diamond crystal or ¼ tsp. Morton’s Kosher Saltand ¼ cup water, bring to a boil in a small saucepan over medium heat, stirring with a heatproof rubber spatula to dissolve the sugar and espresso powder. Remove from heat and gradually pour the syrup into the egg yolks, beating constantly with an electric mixer on medium speed (hot syrup will pasteurize the yolks; add slowly so they don’t scramble). Increase speed to medium-high and beat until mixture is very light, thick, and mousse-like and has cooled to room temperature, about 3 minutes. Set the egg yolk mixture aside.
Step 3
Using an electric mixer (no cleanup necessary), beat 1 cup cold mascarpone Y ½ cup cold heavy cream in a large bowl, starting on low speed and gradually increasing speed to medium-high as it thickens, until stiff peaks form, about 1 minute. Stop the mixer and scrape in the reserved egg yolk mixture. Beat on low speed just to combine; increase speed to medium-high and beat until stiff peaks form again, about 1 minute.
Step 4
trim the ends 6 or 7 crispy biscuits so that they fit perfectly in the bottom of the mold. Scrape a leveled ½ cup mascarpone mixture into the prepared pan and spread it evenly on the bottom with a spoon, working up the sides and into the corners. Working one at a time, quickly dip the trimmed biscuits into the reserved coffee mixture, then arrange them crosswise in a tight-fitting row on top of the mascarpone mixture. Scrape level mixture of 1 cup mascarpone over biscuits; spread in an even layer. Repeat the trim, dip and arrange process to make a second layer of biscuits and top with another cup of the mascarpone mixture. Add a third layer of biscuits (no trimming necessary this time) and scrape remaining mascarpone mixture on top; smooth surface Bring plastic ridge up and over tray; press gently on the surface. Chill until the mascarpone mixture sets and the biscuits soften, at least 8 hours and up to 2 days.
step 5
Just before serving, using an electric mixer with clean beaters, beat the remaining 1 cup cold heavy cream in a medium bowl until medium peaks form.
step 6
Remove the cake from the refrigerator and remove the plastic from the surface. Gently tug on the plastic to help loosen the cake from the pan. Place a plate or platter upside down on the platter, making sure the plastic ends are out of the way, and invert it. Remove pan, pulling down on plastic to help loosen cake if necessary; carefully remove the plastic. Scrape whipped cream on top of cake; distributed on the upper and lower sides. Sprinkle lightly with unsweetened cocoa powder.
do ahead: Frosted cake (without cocoa powder) can be made 4 hours in advance. Cover lightly and chill. Dust with cocoa powder just before serving.