Scrub the potatoes clean with water. Poke a few holes in the potatoes and rub with the olive oil. Place on a baking sheet and bake for about 1 hour.
Remove the potatoes, and reduce the temperature to 375 degrees. When the potatoes are cool enough to touch, slice in 3 sections lengthwise, making sure the 2 end sections are big enough to hold the fillings. Scoop out the interior of the potato and mash in a bowl, along with the milk, sour cream, cheese, butter, and jalapeños. Return the mixture to the potato skin "bowls." Return to the oven and bake for 15 minutes. Garnish with scallions.