Step 1

Mix the cooled espresso with ¾ cup of the Baileys in a shallow bowl. Set aside.

Step 2

Separate the eggs, but keep only one of the whites. In a medium bowl, whisk the egg yolks and sugar together until the mixture is thick, pale yellow, and forms a ribbon when the whisk is lifted out of the bowl, about a minute. Add the remaining ¼ cup Baileys and mascarpone and whisk until the mixture is smooth.

Step 3

In a separate bowl, vigorously whisk the single egg white until soft peaks form when you lift the whisk out of the bowl (you can do this by hand with such a little amount). Fold the egg white into the mascarpone mixture. It will seem thin but don't worry: it will set up in the fridge.

Step 4

Dip the cookies, one at a time, into the espresso/Baileys mixture; let them soak just long enough to become damp but not soggy. Line the bottom of a 9-inch square glass dish (or 7x11-in rectangular dish) with a layer of soaked cookies, then spread half of the mascarpone mixture over top. Repeat with another layer of soaked cookies, then top with the remaining mascarpone mixture. Cover the dish with plastic wrap and leave in the fridge until the mascarpone mixture is set, 8-10 hours or overnight.

Step 5

When you are ready to serve, place the cocoa powder in a small sieve and dust over the top of the tiramisu.