Line 8 sections of a 12-section bun tray with paper cake cases. Beat together the butter and muscovado sugar with a hand-held electric whisk until pale and creamy. Sift together the flour, cocoa powder and mixed spice.
Gradually beat the egg into the butter and sugar mixture, then stir in the flour mixture.
Reserve 2 tablespoons each of the nuts and chocolate and stir the remainder into the cake mixture with the ginger and fruit. Mix well.
Divide the cake mixture between the paper cases. Scatter with the reserved nuts and chocolate and sprinkle with a little demerara sugar.
Bake in a preheated oven, 150°C (300°F), Gas Mark 2, for 35 minutes or until firm to the touch. Transfer to a wire rack to cool and wrap in extra, decorative paper case, if liked.