Beat the mascarpone and caster sugar in a medium mixing bowl with electric beaters until soft peaks form. Set aside.
Combine the coffee and Tia Maria or Kahlua in a large bowl.
Dip 1/2 the Savoiardi biscuits quickly into the coffee mixture and then arrange over the base of a 1.25-1itre (5-cup) serving dish or six 250mls (1-cup) individual serving glasses.
Spread half the mascarpone mixture evenly over the biscuits.
Dip the remaining biscuits into the coffee mixture and then arrange in a single layer over the mascarpone layer.
Finish with a layer of the remaining mascarpone mixture.
Cover with plastic wrap and refrigerate for at least 6 hours or overnight.
Sprinkle with the grated chocolate just before serving.