Preheat the oven to 350�. Butter a 23 cm square baking dish.
In a medium-size saucepan boil the potatoes until tender in water to cover. Drain well and cool. Cut in half and scoop out approximately 2 tsp of potato from each half; discard or reserve for another use.
In a small bowl combine the butter, sour cream, and mustard and blend well. Place a dab of the butter mixture into the cavity of each potato half, then top with the diced ham. Sprinkle with the cheese.
Bake until cheese bubbles, about 10 minutes. Serve hot.