Preheat the oven to 180 C / Gas 4. Line an 18cm x 28cm roasting tin with baking parchment.
Melt dark chocolate and butter in a glass bowl over a pan of simmering water.
Smash the milk and white chocolate into small pieces with a rolling pin (a plastic bag is helpful here).
Whisk the eggs and sugar until thick and frothy.
Pour the melted chocolate and butter mixture over the top of the egg mixture, fold in carefully. Sift the flour and baking powder in and fold in carefully.
Pour the mixture into the lined tin and sprinkle broken chunks of chocolate over the top before baking.
Bake until knife put into cake comes out clean, 20 to 25 minutes.
Can be stored for up to 2 days (unless eaten first!) in an airtight container.