Melt the plain dark chocolate and butter together in a bowl over a pan of simmering water (see
Whisk the eggs with the sugar and maple syrup until pale and light, then stir in the melted chocolate mixture, flour, salt and walnuts, if used.
Grease and line a 30 x 20 cm (12 x 8 inch) baking tin, allowing the paper to overhang the edges of the tin. Spoon the mixture into the tin and scatter the white chocolate over the surface.
Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 35–40 minutes until the top sets but the cake mix still feels soft underneath. Cover the tin loosely with foil if the surface begins to brown too much. Leave to cool in the tin. Dust with icing sugar and cut into squares to serve.