You will need a square baking dish measuring about 16cm (7") or something similar. Or 6-8 serving glasses.
Instructions
Cream and Mascarpone
Place the double cream in a roomy bowl and beat to the soft peak stage.
Using the same beaters, beat the sugar and mascarpone lightly, for about 20 seconds, just to loosen the cheese.
Fold the cream into the mascarpone in 3 additions. That's it for the filling.
Assembling the Tiramisu
Pour the coffee and alcohol, if using, into a shallow dish.
Dip the biscuits into the coffee mix for 5 seconds, no more, shake off excess, and arrange on the base of your chosen dish in a single layer. Or in glasses as in the image here. If using glasses, you'll have to break the biscuits to fit, and push them down.
Top the damp biscuits with half the mascarpone mixture for the big dish. In glasses, about 1-2 heaped tablespoons, depending on the size of your glasses.
Grate half the chocolate all over the mascarpone layer.
Repeat this with another layer of the coffee dipped savoiardi biscuits, the mascarpone mix and the second half of the grated chocolate.
Cover with clingfilm and place in the fridge for a minimum of 4 hours to set, even overnight, for the ideal party dessert.
Just before you serve, top the grated chocolate with a fine layer of cocoa powder using a sieve.