Preheat the oven to 190C/375F/Gas 5 and grease and line a 20cm/8in square baking tin with baking parchment.
Melt the dark chocolate and butter together in a heatproof bowl set over a pan of simmering water (don't let the bottom of the bowl touch the water). Remove from the heat and set aside to cool slightly.
In a mixing bowl, whisk the eggs until light, then beat in the sugar until fluffy.
In a separate bowl, mix the flour, baking powder and walnuts.
Stir the melted chocolate mixture into the eggs with a pinch of salt. Stir in the flour mixture, milk chocolate and cherries.
Pour into the prepared tin and bake in the oven for 40 minutes.
Remove from the oven and leave to cool in the tin.
Cut into squares and serve with clotted cream, raspberries, fresh mint leaves and a dusting of icing sugar.