Combine the marinade ingredients in a bowl or zip-top bag. Add the skirt steak and marinate for at least 1 hour while the potatoes cook; overnight is even better.
Coat the potatoes with olive oil and bake at 425°F for about 1 hour, until fork-tender. Let cool enough to handle.
Halve the potatoes lengthwise. Scoop the flesh into a bowl, leaving a thin layer attached to each skin.
Heat neutral oil to 350°F. Fry the potato skins until golden and crisp, then drain.
Mix the reserved potato flesh with butter, shredded cheese, milk, sour cream, chives, and SPG seasoning until creamy.
Fill each fried potato skin about two-thirds full with the potato mixture.
Grill the marinated steak over high heat, turning every 2–3 minutes, until it reaches your preferred doneness.
Rest the steak for 10 minutes, then slice against the grain into bite-size pieces.
Top the filled skins with steak and additional cheese. Bake at 350°F for about 5 minutes, until warmed through and the cheese melts.
Finish with chipotle sour cream and chives. Serve hot.