1. In a large bowl beat egg yolks and sugar until thick, about 5 minutes.
2. Add mascarpone cheese and beat until smooth.
3. Add 1 tablespoon of espresso and mix until thoroughly combined.
4. In a small shallow dish, add remaining espresso and rum.
5. Dip each ladyfinger into espresso for only 5 seconds. Letting the ladyfingers soak too long will cause them to fall apart.
6. Place the soaked ladyfinger on the bottom of a 13 by 9 inch baking dish, breaking them in half if necessary in order to fit the bottom.
7. Spread evenly 1/2 of the mascarpone mixture over the ladyfingers.
8. Arrange another layer of soaked ladyfingers and top with remaining mascarpone mixture.
9. Cover tiramisu with plastic wrap and refrigerate for at least 2 hours, up to 8 hours.
10. Before serving, sprinkle with chocolate shavings.