1. Heat your oven to 180°C. Line a 1kg loaf tin with baking parchment. For the filling, soak the sultanas in the brandy in a small bowl.
2. To make the cake, cream the butter and sugar together in a large bowl until pale and fluffy; this can take up to 10 minutes. Gradually beat in the eggs and brandy, adding a little of the flour with each egg to guard against curdling. Sift the remaining flour, cocoa and baking powder together over the mixture and fold in gently. Finally, fold in the walnuts and biscuit crumbs. Scrape the mixture into the prepared tin.
3. Bake for about 45 minutes or until firm to touch. Leave in the tin for about 10 minutes, then turn out and cool completely on a wire rack.
4. For the filling, chop 150g of the chocolate and melt in a bowl over a pan of simmering water. Let cool until barely tepid. In a large bowl, cream the butter and the icing sugar together, then vigorously beat in the melted chocolate until creamy. Add the walnuts and sultanas with their brandy.
4. Slice the cake horizontally into 3 layers. Use two-thirds of the filling to sandwich these together and spread the remaining filling on top of the cake. Grate the rest of the chocolate on top and chill the cake thoroughly before serving. Sprinkle with icing sugar to finish.