In a shallow bowl, stir together the cooled espresso (or strong coffee), coffee liqueur (if using), and vanilla extract. Set aside. The mixture should be cool, not warm, so it doesn’t over-soften the ladyfingers.

Fill a medium saucepan with 1–2 inches of water and bring it to a very gentle simmer. Place a heatproof bowl on top, making sure the bottom of the bowl does not touch the water. This acts as a double boiler.

Add the egg yolks and granulated sugar to the bowl over the simmering water. Whisk constantly for 8–10 minutes, until the mixture is pale, thick, slightly increased in volume, and warm (not hot).

Remove the bowl from the heat and whisk for another minute off the stove to release steam. Let cool for about 5 minutes, stirring occasionally to prevent a skin from forming.

Add the cold mascarpone cheese to the lukewarm yolk mixture. Using a hand mixer on low speed or a spatula, gently blend just until smooth and fully combined. Do not overmix to avoid a grainy texture.

In a separate chilled bowl, whip the cold heavy whipping cream to medium-stiff peaks. It should hold its shape but still look soft and smooth, not chunky or curdled.

Gently fold the whipped cream into the mascarpone mixture in 2–3 additions, using an up-and-over motion and turning the bowl as you go. Fold just until no streaks remain and the cream is light and airy.

Have a 9×13-inch baking dish ready. Place the coffee mixture nearby and lay out the ladyfingers so assembly can go quickly.

Working one at a time, quickly dip each ladyfinger into the coffee mixture for about 1 second per side. They should be moist but not collapsing. Arrange in a single layer across the bottom of the pan, trimming to fit as needed.

Spread half of the mascarpone cream evenly over the layer of ladyfingers, smoothing the surface with an offset spatula or the back of a spoon.

Create a second layer of dipped ladyfingers on top of the cream, then spread the remaining mascarpone cream evenly over the top. Smooth the surface as neatly as possible.

Using a fine-mesh sieve, generously dust the top of the tiramisu with unsweetened cocoa powder. Add dark chocolate shavings now or just before serving, if desired.

Cover the dish tightly with plastic wrap and refrigerate for at least 6 hours, preferably 8–24 hours, until fully chilled and set so it slices cleanly.

Just before serving, add an extra light dusting of cocoa powder and more chocolate shavings if you like. Cut into squares with a sharp knife, wiping the blade between cuts for neat slices.