In a large bowl, whisk egg yolks and 1/2 cup sugar until thick and pale, about 3-4 minutes. The mixture should fall in ribbons when lifted.

Add mascarpone to the egg mixture and whisk until smooth and well combined. Set aside.

In a separate clean bowl, whip heavy cream with remaining 1/4 cup sugar until soft peaks form. Don't overwhip.

Gently fold the whipped cream into the mascarpone mixture in three additions, folding carefully to maintain the light texture.

Combine cooled espresso and coffee liqueur (if using) in a shallow dish wide enough for dipping ladyfingers.

Quickly dip each ladyfinger in the coffee mixture, turning to coat both sides but don't let them get soggy. Arrange in a single layer in a 9x13 inch dish or similar.

Spread half of the mascarpone mixture evenly over the first layer of ladyfingers using an offset spatula.

Repeat with another layer of dipped ladyfingers and the remaining mascarpone mixture, smoothing the top.

Cover tightly with plastic wrap and refrigerate for at least 4 hours, but preferably overnight or up to 3 days.

Before serving, dust the top generously with cocoa powder using a fine-mesh sieve. Cut into squares and serve chilled.