Using an electric mixer in a medium bowl, whip together egg yolks and 1/4 cup sugar until very pale yellow and about tripled in volume; a slight ribbon should fall from the beaters, or whisk attachment, when lifted from the bowl.
Transfer mixture to a large bowl, wiping out the medium bowl used to whip the yolks; set aside.
In the medium bowl, whip cream and remaining 1/4 cup sugar until it creates soft-medium peaks.
Add mascarpone and continue to whip until it creates a soft, spreadable mixture with medium peaks.
Gently fold the mascarpone mixture into the sweetened egg yolks until combined.
In a shallow bowl, combine espresso and rum; set aside.
Using a sifter, dust the bottom of an 8 x 8-inch dish, or a 9-inch round cake pan with 1 tablespoon cocoa powder.
Working one at a time, quickly dip each ladyfinger into the espresso mixture; keep in mind, they are quite porous and will fall apart if left in the liquid too long.
Place them, rounded side up, at the bottom of the baking dish; repeat, using half the ladyfingers, until you’ve got an even layer, breaking the ladyfingers in half as needed to fill in any obvious gaps.
Spread half the mascarpone mixture onto the ladyfingers in one even layer.
Repeat with remaining espresso-dipped ladyfingers and mascarpone mixture.
Dust top layer with remaining tablespoon of cocoa powder.
Top with shaved or finely grated chocolate, if desired.
Cover with plastic wrap and let chill in the refrigerator for at least 4 hours or up to 24 hours, all the better, before slicing or scooping to serve.
