For truffles, heat cream in a saucepan over medium-low heat until just below boiling point. Place chocolate in a heatproof bowl and pour over hot cream, stirring until smooth. Stir in liqueur, if using, then chill for 6-8 hours or until firm enough to roll. Use 2 teaspoons or your hands to roll the truffle mixture into about eight 2cm balls, then roll in grated chocolate. Keep chilled in an airtight container for up to 2 days.
For ganache, heat cream over medium-low heat, stirring, until just below boiling point. Place chocolate in a heatproof bowl and pour over hot cream, stirring until smooth. Chill for 6-8 hours until thickened.
Preheat the oven to 160°C. Grease and line the base and sides of three 20cm springform cake pans.
Beat butter and sugar with electric beaters until thick and pale. Add vanilla, then eggs 1 at a time, beating well after each addition. Mix in one-third of the flour, then half the milk. Repeat, then beat in the remaining flour. Gently beat in the melted chocolate until combined.
Divide mixture evenly among pans, then bake for 25-30 minutes until firm to touch and a skewer inserted in the centre comes out clean. (Rotate the pans halfway through cooking time, if necessary, to ensure even baking.) Cool in the pan for 5-10 minutes, then transfer to wire racks, right-way up. Remove baking paper and cool completely.
Once the cakes have cooled, spread 2 cakes with some ganache, leaving three-quarters to decorate, then sandwich the cakes together to form 3 layers. Use the remaining three-quarters of the ganache to cover the top and sides of the layered cake. Decorate with truffles and serve. The iced cake will keep in an airtight container for up to 2 days.