Grease and line the base and sides of a brownie tin. Preheat the oven to 180`c. Break up the 60g plain chocolate and put in a heatproof bowl set over a pan with hot water. Melt down the chocolate gently, remove the bowl from the pan and allow to cool down slightly until needed. Place the butter and icing sugar into a bowl and beat until light and creamy (Alternatively you can use an electric hand whisk). Next, gradually beat in the eggs, followed by the rum. Scrape down the sides of the bowl then beat in the melted chocolate. Stir in the flour and when thoroughly combined stir in the chocolate chips and the nuts. Transfer the mixture to the prepared cake tin and spread out evenly. Bake in a hot oven for 20 to 25 minutes, or until the top is firm. Remove the tin from the oven and allow the brownies to cool down before removing from the tin. Carefully cut the cake into16 pieces using a large sharp knife. Serve warm or at room temperature. Once cool, store in a airtight container and eat consume within 4 days.