In a heatproof bowl, whisk together egg yolks and sugar. Place the bowl over a pot of simmering water (double boiler) and whisk continuously for about 10 minutes until the mixture is pale and thick.

Remove the bowl from heat and let it cool slightly. Add the mascarpone cheese and whisk until smooth and well combined.

In a separate bowl, whip the heavy cream until stiff peaks form. Gently fold the whipped cream into the mascarpone mixture until fully incorporated.

Mix the cooled coffee and coffee liqueur (if using) in a shallow dish. Quickly dip each ladyfinger into the coffee mixture (do not soak) and arrange them in a single layer in a 9x13-inch dish.

Spread half of the mascarpone mixture over the ladyfingers. Repeat with another layer of dipped ladyfingers and the remaining mascarpone mixture.

Cover and refrigerate for at least 4 hours, preferably overnight. Before serving, dust the top with cocoa powder.