White & dark chocolate cake

Name: White & dark chocolate cake
Cook time: 30 min
Total time: 30 min
Cuisine: British
Ingredients: 175g butter softened, plus extra for greasing; 100g white chocolate; 100g dark chocolate (70% cocoa); 3 eggs; 100ml whole or semi-skimmed milk; 175g caster sugar; 1 tsp baking powder; 200g self-raising flour; 2 tbsp very strong coffee (instant is fine); 1 tsp vanilla extract; 284ml pot double cream; 200g dark chocolate (70% cocoa); 50g white chocolate melted; small bag white chocolate coated malt balls (optional)