For the truffles, melt 115g/4oz of the plain chocolate and 115g/4oz of the white chocolate in separate bowls.
Meanwhile, heat the cream until just boiling. Divide the cream between the plain chocolate and white chocolate and whisk each mixture until thick and glossy.
Leave to chill in the fridge for 2-3 hours.
To make the truffles, melt the remaining plain chocolate and white chocolate in separate bowls. Line a baking tray with greaseproof paper.
Scoop out two teaspoons of the white chocolate truffle mixture, roll into a ball, carefully dip into the melted plain chocolate and set aside on the baking tray. Repeat with the remaining white chocolate truffle mixture.
Repeat Step 5 with the plain chocolate truffle mixture and dip the truffles into the melted white chocolate.
Meanwhile, for the marble cake, preheat the oven to 180C/350F/Gas 4. Grease and flour two 15cm/6in sandwich tins and two 20cm/8in tins.
Beat the butter, sugar and eggs together in a large bowl until well combined. Fold in the flour.
Divide the mixture between the four tins, then pour a little melted chocolate into each tin. Swirl the cake mixture with a skewer to create a marbled effect.
Bake the 15cm/6in cakes for 30-40 minutes and the 20cm/8in cakes for 40-50 minutes, or until a skewer inserted into the middle of each cake comes out clean. Remove the cakes from the oven and set aside to cool completely.
Meanwhile, for the buttercream, beat the butter and icing sugar together in a bowl until light and fluffy. Beat in the melted chocolate and milk. Set aside.
For the chocolate ganache, whisk 175ml/6fl oz of the whipping cream into the melted plain chocolate until smooth and glossy.
Whisk the remaining whipping cream into the white chocolate until smooth and glossy.
To assemble the cakes, cut each 15cm/6in cake in half horizontally and fill with buttercream to make a four layer cake. Spread a little buttercream onto the 15cm/6in cake board and place the cakes onto them.
Repeat Step 14 with the 20cm/8in cake.
Ice the 15cm/6in cake with the plain chocolate ganache and the 20cm/8in cake with the white chocolate ganache. Chill in the fridge for 1-2 hours, or until the ganache has hardened.
To stack the cakes, hold a dowelling rod at the side of the 20cm/8in cake and mark with a pencil where the icing comes to. Cut the rod and three others to the same length. Push the rods into the cake about 5cm/2in away from the sides, to form the four corners of a square in the centre of the cake and top with the 15cm/6in cake.
Decorate with the chocolate truffles and chocolate shavings.