To make the icing, beat the butter until pale. Sieve in the icing sugar in batches, beating well between each addition. Beat in the soft cheese and maple syrup. Swirl the icing over the top of the cake with a palette knife. Decorate with the orange zest, a drizzle of maple syrup and a sprinkling of nuts, if using. Cut the cake into 12 squares. Will keep for three days in an airtight container in the fridge.