Preheat oven to 200°C/180°C fan-forced. Lightly spray a large baking tray with oil.
Place potatoes on a plate lined with paper towel. Sprinkle potatoes with a little water and cover with another paper towel. Microwave on High (100%) for 5 to 6 minutes or until just tender.
Carefully arrange hot potatoes on prepared tray. Using a potato masher, gently squash each potato. Combine oil, butter and VEGEMITE in a bowl and whisk until combined. Brush potatoes with VEGEMITE mixture and season with pepper. Roast for 20 minutes.
Sprinkle potatoes with garlic. Roast for a further 10 minutes or until potatoes are golden and tender. Sprinkle with chives. Serve.