1. Light large chimney starter filled with charcoal (6 quarts, or about 100 briquettes) and allow to burn until coals are fully ignited and covered with thin layer of ash, about 20 minutes. Empty coals into grill; build two-level fire by arranging two-thirds of coals over half of grill and arranging remaining coals in single layer over other half. Position cooking grate over coals, cover grill, and heat grate until hot, about 5 minutes; scrape grate clean with grill brush. Remove lid and let coals burn until fire on hotter part of grill is medium (you can hold your hand 5 inches above grate for 5 to 6 seconds), about 10 minutes. Dip wad of paper towels in vegetable oil; holding wad with tongs, wipe cooking grate.
2. Meanwhile, heat olive oil, garlic, rosemary, and 1/2 teaspoon salt in small skillet over medium heat until sizzling, about 3 minutes. Reduce heat to medium-low and continue to cook until garlic is light blond, about 3 minutes. Pour mixture through fine-mesh strainer into small bowl; press on solids. Measure 1 tablespoon solids and 1 tablespoon oil into large bowl and set aside. Discard remaining solids but reserve remaining oil.
3. Place skewered potatoes in single layer on large microwave-safe plate and poke each potato several times with skewer. Brush with 1 tablespoon strained oil and season liberally with salt. Microwave on high power until potatoes offer slight resistance when pierced with tip of paring knife, about 8 minutes, turning them halfway through cooking time. Transfer potatoes to baking sheet coated with 1 tablespoon strained oil. Brush with remaining tablespoon strained oil; season with salt and pepper to taste.
4. Place potatoes on hotter side of grill. Cook, turning once, until grill marks appear, about 4 minutes. Move potatoes to cooler side of grill; cover with disposable pan and continue to cook until paring knife slips in and out of potatoes easily, 5 to 8 minutes longer. Remove potatoes from skewers and transfer to bowl with reserved garlic/oil mixture; add chives and toss until thoroughly coated. Serve immediately.
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