2.To make the base, combine the biscuit crumbs, condensed milk, butter, walnuts and coffee in a large bowl and mix together well. Add the chocolate chips then press the mixture into the pan. Refrigerate the base for 1 hour.
3.To make the icing, combine the icing sugar, butter and coffee and mix together well. Add sufficient hot water to make the mixture a spreadable consistency.
4.Spread the icing over the base and sprinkle over the extra walnuts and chocolate chips, gently pressing them into the icing. Chill the slice until it has set. Remove the slice from the pan and cut it into square pieces.