In a heatproof bowl, whisk together egg yolks and sugar until pale yellow and slightly thickened. Place the bowl over a pot of simmering water (double boiler method), making sure the bottom doesn't touch the water. Slowly add milk while constantly whisking. Continue cooking for about 10 minutes, whisking constantly, until the mixture thickens enough to coat the back of a spoon. Remove from heat and let cool completely.

In a large bowl, beat the mascarpone cheese until smooth. Gradually fold in the cooled egg custard until fully incorporated. In a separate bowl, whip the heavy cream and vanilla extract until stiff peaks form. Gently fold the whipped cream into the mascarpone mixture until smooth and uniform. Be careful not to overmix.

In a shallow dish, combine the cooled coffee and coffee liqueur.

Quickly dip each ladyfinger into the coffee mixture (about 1 second per side—don't soak them) and arrange them in a single layer in the bottom of a 9×13-inch baking dish. Spread half of the mascarpone mixture evenly over the ladyfingers. Repeat with another layer of dipped ladyfingers and top with the remaining mascarpone mixture.

Using a fine-mesh sieve, dust the top with cocoa powder. If desired, add chocolate shavings for extra decoration.

Cover the tiramisu with plastic wrap and refrigerate for at least 2 hours, preferably overnight, to allow the flavors to meld and the dessert to set properly.