Place the cream in a bowl and beat until the cream forms soft peaks.
In a separate bowl (medium-large), cream together the mascarpone and sugar until the mixture is smooth and glossy. Fold in the whipped cream so everything is well mixed but still relatively light (try not to lose all the air). Set aside, ideally in the fridge until you are ready to use.
Cut the panettone into fingers, roughly the size of ladyfinger cookies (so around ⅓ in/1cm deep, 1 in/2.5cm wide). Place the coffee in a shallow dish, wide enough to hold a piece of the panettone but not too much more.
Dip slices of panettone into the coffee then place them in a single layer in a dish or dishes (I'd suggest one dish approximately 8x6in, 20x15cm and 2in/5cm deep or equivalent volume, or you could make in individual dishes).
Spread roughly half of the mascarpone-cream mixture over the panettone slices and spread to form an even layer, spreading right to the edges. If you like, grate some chocolate over the cream layer.
Dip the rest of the panettone pieces in the coffee and form a second layer on top of the cream, then top this with the rest of the mascarpone cream.
Spread the mascarpone cream mixture evenly and to the edges and smooth off the top. Top with the cocoa powder, sprinkled on through a fine mesh sieve.
Chill the tiramisu, covered, in the fridge for a few hours or, even better, overnight, before serving.