Cut a thin slice off the bottom off the potatoes and stand them on end in a muffin tin (they'll stand up easily since you've taken a slice off the bottom). Brush the skins generously with melted butter, and bake in a preheated moderate oven (Mk 4 - 350°F - 180°C) for 25 minutes.
Meanwhile, make alternate layers of ham and cheese, (5 layers of cheese and 4 layers of ham works well). Use a small glass to cut circles out of the ham and cheese. You'll need 6 circles about 1½" to 2" - 4-5cm -in diameter. Add chopped chives to the sour cream, and sprinkle with salt and pepper. If you have ham left over from cutting out the circles, chop into tiny pieces and stir into the cream.
When the potatoes are cooked, cut off the tops and scoop out the flesh (be sure not to go too deep and make a hole in the base). Spoon in the sour cream mix leaving enough space at the top for a ham and cheese circle. Place the ham and cheese circles on the top to roughly 'seal' the hole. Top with grated mozzarella and bake in the oven again for 10 minutes at the same temperature (Mk 4 - 350°F - 180°C). Serve straight away.