Fill a medium saucepan with about 1–2 inches of water and bring to a gentle simmer over medium-low heat. In a medium heatproof bowl (metal or glass) whisk together the 4 egg yolks and 1 cup granulated sugar until smooth and slightly pale. Place the bowl over the simmering water (bowl should not touch the water) to create a double boiler. Whisk constantly for 8–10 minutes until the mixture thickens and becomes ribbon-like when the whisk is lifted (zabaglione). Remove from heat and let cool slightly for 5 minutes.

Incorporate mascarpone

While the zabaglione is still warm but not hot, gently whisk in the 16 oz mascarpone cheese in two additions, mixing until smooth and lump-free. If the mascarpone is very cold and lumps resist, let it sit at room temperature 10–15 minutes before beginning.

Whip cream and fold

In a separate chilled bowl, whip 1 cup heavy cream with 1 tsp vanilla extract and a pinch of salt to medium-stiff peaks. Fold one-quarter of the whipped cream into the mascarpone mixture to lighten it, then gently fold in the remaining whipped cream until uniform and airy. Refrigerate the mascarpone cream until assembly (at least 15 minutes).

Coffee Soak

Prepare 1.5 cups strong hot espresso or very strong brewed coffee. Stir in 3 tablespoons coffee liqueur if using and 1 teaspoon sugar only if you prefer a slightly sweeter soak. Allow the coffee mixture to cool to room temperature — it should be warm or cool, not hot, before soaking ladyfingers to avoid sogginess.

Assembly

Prepare an 8x8-inch or 9x9-inch (20x20 or 23x23 cm) dish. Quickly dip each ladyfinger into the coffee mixture for 1–2 seconds per side — do not soak; they should be moistened but still hold shape. Arrange a single layer of 12 ladyfingers (or as needed) in the bottom of the dish, trimming to fit if necessary.

Spread half of the mascarpone cream evenly over the soaked ladyfingers, smoothing the top with an offset spatula.

Add a second layer of dipped ladyfingers over the cream, again dipping each 1–2 seconds and arranging snugly. Spread the remaining mascarpone cream over the second layer and smooth the surface.

Cover the dish with plastic wrap and refrigerate at least 6 hours, preferably overnight, to allow flavors to meld and the cream to firm up.

Just before serving, sift 2 tablespoons unsweetened cocoa powder evenly over the top. Optionally shave 1 oz dark chocolate over the cocoa for extra texture and garnish.

Cut into squares and serve chilled. Keep refrigerated; best within 2–3 days.