Prepare the custard: In a medium saucepan, whisk together egg yolks and sugar until well blended. Whisk in milk and cook over medium heat, stirring constantly, until mixture boils. Boil gently for 1 minute, then remove from heat and allow to cool slightly.

Whip the cream: In a medium bowl, beat heavy cream with vanilla extract until stiff peaks form.

Combine custard and mascarpone: Whisk mascarpone into the yolk mixture until smooth. Gently fold in the whipped cream.

Prepare the coffee mixture: In a shallow dish, combine brewed coffee and coffee liqueur (if using).

Assemble the tiramisu: Quickly dip each ladyfinger into the coffee mixture and arrange 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.

Chill and serve: Cover and refrigerate for at least 4 hours, or overnight. Before serving, dust with cocoa powder.