Preheat the oven to 170°C and grease and line 23cm and 15cm cake tins.
Cream butter and sugar until light and fluffy, then beat in eggs one at a time.
Fold in flour and cocoa powder, then stir in melted dark chocolate.
Divide the batter between the two prepared tins.
Bake the larger cake for 60-75 minutes and the smaller cake for 45 minutes.
Cool cakes in tins briefly before transferring to a wire rack to cool completely.
Cream butter, vanilla, and icing sugar for the white buttercream, then stir in melted white chocolate.
Repeat the previous step using dark chocolate to create the dark chocolate buttercream.
Bring cream to a boil and pour over chopped white chocolate, stirring until smooth for the ganache.
Halve cakes horizontally, fill with dark buttercream, and place onto boards using a dab of icing.
Coat cake exteriors with white buttercream and chill for 30 minutes.
Apply ganache smoothly over both cakes and chill again for 30 minutes.
Stack cakes using trimmed dowelling rods and pillars, then decorate with fresh flowers.