Melt butter in saucepan, sauté leeks and onions slowly until tender but not brown. Add chicken broth, parley, celery, mace, potatoes and salt. Cover, simmer until tender. Remove from heat. With a hand blender puree mixture moving blender up and down and sideways through potato mixture. Add cream and milk; blend. Return saucepan to stove; heat mixture just to boiling point. Cool, then refrigerate for several hours before serving. Serve in individual bowls. Top with chopped parsley.Serves 8 Add mace only if desired.