Heat the oven to 170C/fan 150C/gas 3. Butter and line the base of a 20cm springform tin. For the torte, break the chocolate into a bowl set over a pan of gently simmering water, and add the butter. Heat until melted, than take off the heat and stir until smooth. Cool for about 5 mins.
Stir in the egg yolks and ground almonds. Whisk the egg whites to soft peaks, then start to add the sugar, a spoonful at a time, until stiff peaks form. Stir a spoonful or two of the egg whites into the chocolate mixture to slacken it, then carefully fold in the rest.
Spoon the mixture into the prepared tin and bake for 30-35 mins until risen and just firm. Cool in the tin before turning out – don’t worry if it cracks and sinks a bit. Peel off the lining paper.
For the ganache, put the chocolate in a heatproof bowl. Heat the cream to just below boiling point, then pour onto the chocolate, stirring. When the chocolate has melted, add the butter and stir until the mixture is glossy and smooth. Cool slightly. Cut the cake in half and sandwich together with a third of the ganache. Use the rest to cover the top and sides of the cake.
To decorate, melt the milk and white chocolates separately, either in a small bowl over a pan of simmering water or on medium in the microwave in 30 sec bursts. Drizzle alternately on top of the torte. The torte will keep in an airtight container in the fridge for up to one week.