Heat the oven to 400 degrees Fahrenheit.
Scrub and dry the potatoes, making sure to get into all of the little crevices. Place them onto a foil- or parchment-lined baking tray. Poke some holes in the potatoes with a fork, which will allow the steam to escape. Top with a drizzle of oil and a sprinkle of salt.
Let them bake for an hour. The skin should look crispy, and a sharp knife should easily slide into the center. Remove them from the oven.
When the potatoes have cooled enough to touch, use a sharp knife to slice a long oval on the top. Then, scoop out the insides with a spoon, leaving a thin layer of roasted potato on the skin.
In a medium bowl, mash the potatoes with butter, mascarpone, and cheese, then add chives. Add salt and pepper to taste.
Load each potato skin with the filling.
If serving immediately, bake the potatoes for five to 10 minutes on the center rack. Broil them on high for another five to seven minutes, making sure they don't burn.
If your potatoes are for the following day, then refrigerate them in an airtight container. When you want to serve them, bake the potatoes for 15 to 20 minutes at 400 degrees Fahrenheit and follow the previous instructions for broiling.
