Wash the potatoes thoroughly and grate them dry. Wrap in aluminium foil and place on the fat pan of the oven. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 45-50 minutes.
Take out potatoes, remove aluminium foil. Let the potatoes cool down a little. Cut off about 1/4 of the potatoes lengthwise. Hollow out 4 potatoes to a 1 cm thick edge. Remove the skin from the "lids" and the remaining potatoes.
Press everything except the 4 hollowed out potatoes through a potato ricer. Add butter, egg, cheese and soured milk and mix. Season to taste with salt, pepper and nutmeg. Pour into a piping bag with a large star-shaped spout.
Stuff hollowed out potatoes with it. Place in a greased casserole dish. Bake in a preheated oven at the same temperature for about 10 minutes. Cut salmon ham into strips. Wash the chives, dab dry and cut into rings.
Sprinkle chives and ham over the finished potatoes.