Scrub potatoes and poke top a couple of times with a fork. Place on a plate and microwave for 10-20 minutes (depending on size) or until tender. *Test with a sharp knife that you slide through the side of the potatoes. It should pass through easily, without resistance.
(Alternately, you can bake the potatoes in a 425F oven on a baking sheet for 45-60 minutes).
Remove and allow to sit until cool enough to handle, 10-15 minutes. Slice potatoes in half lengthwise. Allow to cool further. *Make ahead! Once mostly cooled, place a bit of plastic wrap over just the top/cut side of the potato and refrigerate until later.
Preheat oven to 450F.
Score potato flesh by cutting diagonal lines only through the flesh (not the skin!), then repeating making diagonal lines in the other direction, to make a diamond pattern on top of the potatoes. Place potatoes on a baking sheet.
In a small microwaveable bowl, combine the olive oil and garlic. Microwave for 1 minute. Set aside. In another small bowl, combine the malt vinegar and salt. Microwave for 45 seconds, then remove and stir mixture to dissolve salt. Add the vinegar mixture to the olive oil mixture. Whisk with a fork to combine well.
Generously brush tops of scored potatoes with oil and vinegar mixture.
Place into preheated oven and bake for 10 minutes. Remove from oven. Re-whisk vinegar mixture, then generously brush top of potatoes again. Return to oven for another 10 minutes. Remove from oven. Re-whisk vinegar mixture, then generously brush tops again, but before returning to oven, flip potatoes over, so the cut side is down. Bake a further 10 minutes, or until lovely and golden underneath.
Remove from oven and give potatoes one final brush with vinegar mixture and sprinkle with finishing salt, if using.
Be sure to read the "Cook's Notes" in the original post, for tips, tricks and substitution ideas.