Brew a strong cup of coffee and let it cool completely. If using coffee liqueur, mix it in with the cooled coffee.

In a medium saucepan, whisk together egg yolks and granulated sugar until well combined. Add milk and cook over medium heat, stirring constantly until thickened (about 5-7 minutes). Do not let it boil. Remove from heat and let cool to room temperature.

Whip the heavy cream in a separate bowl until stiff peaks form. Once the custard has cooled, gently fold in the mascarpone cheese until smooth, then fold in the whipped cream until fully incorporated.

Dip each ladyfinger into the coffee mixture quickly, ensuring they are soaked but not soggy. Set aside on a plate.

In a rectangular dish, arrange a layer of soaked ladyfingers at the bottom. Spread half of the mascarpone mixture over the ladyfingers. Repeat with another layer of soaked ladyfingers followed by the remaining mascarpone mixture.

Cover the dish with plastic wrap and refrigerate for at least 4 hours, preferably overnight. Before serving, dust the top with cocoa powder and garnish with dark chocolate shavings.