Preheat oven to 400 degrees Fahrenheit.

Pierce potatoes with a sharp knife. Place on a baking dish and brush with two tablespoons olive oil. Bake for one hour, then let cool.

Heat three tablespoons oil in a skillet. Add spinach and cook, tossing intermittently, for five minutes, until wilted. Add garlic, cayenne pepper, and salt. Increase heat and cook until liquid evaporates, about two minutes.

Halve the potatoes and carefully spoon the flesh into a bowl. Mash potato flesh with remaining three tablespoons oil and season with salt. Add egg yolks, Parmesan, spinach, and sour cream and mix to combine. Fill potato shells and bake for 15 minutes.

Sprinkle potatoes with mozzarella and bake until cheese is melted.