Cut panettone into 1 x 5-in. (2.5 x 12-cm) "fingers". Set aside. In bowl, beat whipping cream to stiff peaks. In separate large bowl, beat mascarpone cheese with icing sugar until light and fluffy. Gently fold whipped cream into mascarpone mixture until combined.
Place the coffee in a shallow dish. Working with half the panettone fingers for the first layer, quickly dip each one in the coffee then lay them, side by side, in bottom of 9 x 13-in. (3-L) baking dish (or after laying panettone fingers in dish, brush with coffee). Spread half the mascarpone mixture evenly over top. Repeat the two layers once more. Finely grate chocolate overtop.
Cover and refrigerate at least 4 hr. or overnight.
Spike coffee with amaretto, sambuca, coffee or chocolate liqueur.
Substitute panettone with 36 ladyfingers (18 per layer) and use 3 cups (750 mL) brewed coffee for dipping the ladyfingers, also called savoiardi cookies.
For a spiked tiramisu, with use 1 1/2 cups (375 mL) coffee mixed with 1/2 cup (125 mL) Amaretto, Sambuca, coffee-flavoured or chocolate liqueur for moistening the panettone.