In a large bowl, whisk egg yolks and 1/2 cup sugar until thick and pale, about 3-4 minutes. Add mascarpone and vanilla extract, whisking until smooth and well combined.

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, being careful not to deflate the mixture.

Combine cooled espresso and coffee liqueur (if using) in a shallow dish. Quickly dip each ladyfinger into the coffee mixture, turning once, about 2-3 seconds per side.

Arrange half of the dipped ladyfingers in a single layer in the bottom of a 9x13 inch dish or individual serving glasses.

Spread half of the mascarpone mixture over the ladyfingers, smoothing with an offset spatula.

Repeat with remaining ladyfingers and mascarpone mixture to create a second layer.

Cover with plastic wrap and refrigerate for at least 4 hours, preferably overnight.

Before serving, dust the top evenly with cocoa powder using a fine-mesh sieve. Serve chilled.