Heat oven to 400ºF (200ºC).
Poke holes with a fork all over the potatoes then rub with the olive oil and season with salt and pepper. Arrange on a baking sheet and bake until the insides of the potatoes are soft, about 1 hour, rotating half way through. Remove from the oven and let cool.
Slice the top third of the potato off, lengthwise. Use a spoon to scoop out the inside of the potato, leaving about a ¼-inch border of potato to form small boats.
Add the flesh of the potatoes to the bowl of a stand mixer fitted with a paddle attachment. Be sure to scoop the flesh of the tops of the potatoes as well then discard the tops.
Add the milk, sour cream and cheese, mixing until just smooth. Season with salt and pepper then add 1 tablespoon of the chives.
Either mound the filling into the potato boats or for a smoother, mound half the mixture into the boats and add the remaining half to a pastry bag fitted with a large star tip and pipe the remaining mixture onto the top of the potatoes. Gently brush the tops with butter.
Lower oven to 375ºF (190ºC). Arrange the filled potatoes on a baking sheet and bake until the filling is heated through and the tops are browned, about 20 minutes. Top with the remaining chives.