Cook potatoes in boiling water for 8 to 12 minutes until just tender. Drain and cool. Slice off the bottom end of each potato so it sits upright and slice off a third of the top. (If potaotes are medium small cut them in half and use both halves) With a melon baller, scoop out the potato and fill with a dollop of sour cream. Top with walnut halves and tuck in a sprig of dill.