Bake the potatoes with a knife incision all around, about 1 hour 20 minutes until fork tender.
Scoop out the potato flesh into a bowl, being careful not to break the skins.
Rice or mash the potato flesh, then add the butter, 4 egg yolks, rosemary salt, black pepper, parsley, and chives. Fold to combine.
Pipe or spoon the potato mixture back into the potato skins, creating a border.
Make an egg wash with the 2 reserved egg yolks and milk, and brush the potato skins.
Top with the grated gruyere cheese.
Broil the stuffed potatoes for 3-4 minutes until the cheese is melted and bubbly.
