Melt the dark chocolate and butter in a heatproof bowl set over a pan of barely simmering water. Alternatively, melt in the microwave using 30 second intervals, stirring till smooth.
Beat the muscovado sugar and eggs together, then stir in the melted chocolate and butter mixture. Add the walnuts, chopped chocolate and flour, mix till just combined. Pour into a greased 28x20cm (11x8") tin.
Bake in a preheated 200 C / 180 C Fan / Gas 6 oven for 30 minutes. To check if done, the middle should be springy and the outer edge just crusting.