Whisk the egg yolks and sugar in a bowl for 2 minutes or until well mixed.
Make a bain-marie, or water bath with a saucepan filled halfway up water. Place the bowl so it rests on the saucepan, but not on the water. Bring the water to a boil and reduce heat to a slow boil. Whisk the egg and sugar mixture constantly for about 8 minutes or until the eggs reach a temperature of 160° F.
Remove eggs from heat and allow to cool about 5-10 minutes.
Meanwhile using a hand mixer, whip the mascarpone for about 2 minutes. Add egg mixture and whip another minute, or until combined.
Combine espresso and hazelnut liqueur together in a shallow dish.
Dip each ladyfinger briefly (count to 3) in espresso mixture and place them in a layer on the bottom of an 8" x 8" baking dish. Break biscuits to fit.
Spread half of the mascarpone cream over the top of the biscuits
Sprinkle crushed, candied hazelnuts over the top of the cream.
Repeat with a second layer of ladyfingers, laying them in the opposite direction. Break the biscuits in half in order to fit them in the baking dish.
Top with remaining mascarpone cream. Cover with plastic wrap and refrigerate for a minimum of 4 hours. (Over-night is better)
Just before serving dust with unsweetened cocoa powder and decorate with remaining candied hazelnuts if desired.