To make cake : Preheat oven to 160oC, Gas Mark 4. Lightly grease and line a 30 cm baking tin with greaseproof paper. In a saucepan warm the caster sugar, golden syrup and milk just so the sugar's dissolve. Leave to one side to cool. Mix together the flour and baking powder in a bowl, add in the chopped walnuts, raisins and sultanas. Add the cooled syrup mixture into the flour and beat until smooth. Add in the eggs and mix. Pour into the lined baking tin. Cook in the preheated oven for 30 minutes, turn and cook for a further 30 minutes or until firm to the touch and when a skewer is inserted into the centre it comes out clean. If the cake looks to be going too brown cover with tin foil and continue cooking. Leave the cake to cool for 10-15 minutes when it comes out of the oven then turn out onto a wire rack. Leave to cool completely before frosing. To make the frosting : Beat the icing sugar and butter until pale and smooth. Gradually add in the cream cheese. Once all incorporated gradually add in the coffee essence to taste. Take the walnut cake and spread the coffee frosting over the top of the cake using a pallet knife. Cut into desired sized portions and garnish with a roasted walnut on top.