Preheat oven to 400° F.

Scrub the potatoes clean, removing all dirt and any eyes (stems starting to grow). Pierce potatoes a few times with a sharp knife. Place potatoes on a cookie or baking tray, and bake for 1 hour, or until tender when pierced with a fork. Remove potatoes from the oven, and let cool on the counter.

While the potatoes bake, prepare milk mixture. Place butter in a large (eventually all the potato insides will be added to this bowl, so make it big) microwave safe bowl. Microwave the butter in 30-second increments, stirring after each time, until butter is melted. Add cream cheese, sour cream, milk, Parmesan, Cheddar, salt, and pepper, stirring well to combine.

When the potatoes are cool enough to handle, cut them in half lengthwise, and scoop out the inside, leaving about a ¼ inch of potato skin and flesh around the edges of the potatoes. Place the scooped out flesh in the large bowl with the milk mixture, or press the potato through a potato ricer into the milk mixture. Stir well to combine, smashing any large chunks of potatoes.

Place the potato skins on a large baking tray or a 13x9-inch baking dish, and spoon the potato mixture into the skins. Potato mixture will mound up out of the skins. Sprinkle with additional Cheddar cheese if desired. Potatoes may be made ahead to this point and kept in the fridge until ready to bake.

When ready to bake, preheat oven to 350° F, and bake twice baked potatoes for 20 minutes. Serve hot.