250g/9oz caster sugar
50g/2oz cocoa powder
200g/7oz self-raising flour
1 tsp vanilla extract
200g/7oz dark chocolate (at least 70% cocoa solids), chopped into small pieces
10 white marshmallows
250g/9oz unsalted butter, softened, plus extra for greasing
200ml/7fl oz double cream
4 free-range eggs, lightly beaten