To make the ladyfinger cookies:

1. Preheat the oven to 350 degrees and line two baking sheets with parchment paper. Fit a large (16-18 inch) pastry bag with a large round tip (I use an Ateco #808) and set aside.
2. Sift the flour and cornstarch together into a small bowl and stir in the espresso powder. Set aside.
3. Add the 6 room temperature egg whites to a large, clean mixing bowl. Add the egg yolks to a smaller bowl and set aside.
4. Measure out half (1/2 cup) of the caster/superfine sugar and place it near the bowl of egg whites so that you can easily grab it.
5. Using a hand mixer with clean beaters, beat the egg whites on medium high speed until very soft peaks form. When you stop the mixer and pull the beaters up, the peaks should sink down a little and not hold up well. 
6. Reduce the mixer speed to medium low and gradually add the 1/2 cup of caster sugar in small additions. Mix each one in completely before adding another. I like to add a tablespoon at a time and mix for about 5 seconds before adding more.
7. After all the sugar has been added, increase the mixer back to medium high and beat the egg whites until they form stiff, glossy peaks. When you raise the beaters up, the peaks should not move much, if at all. Set the egg whites aside, and leave the beaters in the hand mixer without cleaning them.
8. Add the remaining 1/2 cup of caster sugar to the egg yolks, along with the salt and teaspoon of vanilla extract. Use the hand mixer to beat the yolk mixture on high speed until it becomes thicker and pale, about 2-3 minutes.
9. Drizzle the egg yolk mixture over the egg whites and add the flour mixture. Use a spatula to gently fold the batter together. It will start out dry and chunky, but be patient and don’t get rough! Keep the folding motion gentle and rotate the bowl with each fold. The batter will come together in a few minutes, and have a lot of air holes. It won’t look like a smooth cake batter, which is fine! If you have a few stray egg white streaks, that’s okay. 
10. Carefully transfer the batter to the prepared piping bag, and pipe the cookies into 3 inch long logs on the prepared baking sheets. You only need to leave about an inch between the cookies, since they rise rather than spread.
11. After you’ve piped out all the cookies, dust them generously with the 1/2 cup of powdered sugar right before going into the oven. This gives the ladyfingers a much better rise! Don’t add the powdered sugar until immediately before you bake them. 
12. Bake the cookies for 18 minutes, or until light golden brown on the top and sides. They may crackle a bit on the top, which is normal.
13. Allow them to cool on the baking sheet for about a half hour before transferring to a cooling rack. Leave them out on the cooling rack overnight to dry out before using in the tiramisu.

To make the filling:

1. Before you start making the zabaglione, take the mascarpone out of the fridge and scoop it into a large mixing bowl. It will soften enough while you make the custard. 
2. Add water to a 3 quart saucepan until it’s a few inches deep. Set the pan over medium heat until it reaches a simmer. Don’t allow the water to boil.
3. In a small, heatproof bowl that fits onto the saucepan, add the egg yolks and whisk in the caster sugar until the mixture is smooth. (Alternatively, use a dedicated double boiler to make the custard.)
4. Set the bowl on top of the saucepan of simmering water and whisk constantly until the eggs have thickened into a pale yellow custard. This took me about 7 minutes, but it can take anywhere from 6-8 minutes. Remove the bowl from the saucepan and set aside to cool for a few minutes.
5. Use a hand mixer to beat the mascarpone on low speed for about 30 seconds, just to loosen it up a bit. Add the 1/4 cup of rum/coffee liquor and beat on low speed for another 30 seconds until smooth. Add the custard and mix on low again until it is fully incorporated into the mascarpone. Don’t use anything higher than low speed, since mascarpone can separate and curdle easily.
6. Set the bowl into the fridge while you whip the cream.
7. Add the heavy cream to a chilled metal bowl. (if you set a bowl in the freezer for 5 minutes before continuing, that’s totally fine. Gives the custard/mascarpone more time to chill.)
8. Whip the cream on medium speed until slightly thickened (think shampoo) and then increase the speed to high. Whip until very firm peaks form.
9. Fold the whipped cream into the custard/mascarpone mixture until well blended and smooth. Refrigerate until you’re ready to assemble the tiramisu.

To assemble the tiramisu: 

1. Stir the cold brew coffee and liquor together in a wider, shallow bowl. Soak each ladyfinger cookie for a few seconds per side, and line the bottom of a 9 x 13 pan with soaked cookies.
2. Spread half of the filling over the ladyfingers and smooth it out into an even layer. Soak the remaining ladyfingers and arrange them on top of the filling. Add the remaining filling to the top of the second layer of cookies and smooth it out.
3. Dust the tiramisu with the cocoa powder, cover the dish, and refrigerate for at least 8 hours, preferably overnight. Finish up any leftovers within a week.