Melt 115g of plain chocolate and 115g of white chocolate in separate bowls for the truffles.
Boil the cream, divide it between the melted chocolates, and whisk until glossy.
Chill the truffle mixtures in the fridge for 2-3 hours.
Melt the remaining chocolate for coating and line a tray with greaseproof paper.
Roll white chocolate mixture into balls, dip into melted plain chocolate, and place on the tray.
Roll plain chocolate mixture into balls, dip into melted white chocolate, and place on the tray.
Preheat the oven to 180°C and grease four cake tins (two 15cm and two 20cm).
Beat butter, sugar, and eggs together, then fold in the flour.
Divide batter between tins, add melted chocolate, and swirl with a skewer.
Bake the small cakes for 30-40 minutes and large cakes for 40-50 minutes.
Beat butter and icing sugar, then mix in melted chocolate and milk for the buttercream.
Whisk 175ml of whipping cream into melted plain chocolate for the dark ganache.
Whisk the remaining cream into white chocolate for the white ganache.
Split 15cm cakes, fill with buttercream, and stack on the 15cm board.
Split 20cm cakes, fill with buttercream, and stack on the 20cm board.
Ice the small cake with plain ganache and the large cake with white ganache, then chill.
Insert four trimmed dowelling rods into the 20cm cake and stack the 15cm cake on top.
Decorate the assembled cake with truffles and chocolate shavings.