Roast the potatoes for 45-60 minutes until the potatoes are soft when pierced by a knife.
Lay each potato lengthwise and cut a 1 cm deep strip off each top.
Remove the potato flesh taking care not to break the skin. Place the flesh into a bowl and set the potato casing aside. Keep the oven on.
Put the potato flesh into a saucepan and add the cream and butter. Turn the heat onto low and gently mix together until the butter is melted and everything is incorporated.
Flatten the mix with a fork to ensure no hard bits of potato remain, season with salt and pepper and a small grating of nutmeg. Turn off the heat and mix everything in with a wooden spoon one more time.
Make the savory custard by mixing one tablespoon of cream, ½ an egg yolk and the cheese together.
Add the chives and parsley to the potato flesh mixture and stir in, filling each of the potato casings.
Form a furrow along the centre of the flesh and fill it with the custard.
Place the potatoes under a grill for 5 minutes to color the custard, or return them to the oven for 10 minutes at 200C (400F) to warm the flesh and brown the custard.