Bring a pan of salted water to the boil and par boil potatoes for 12 to 14 minutes, then drain the water out of the pan and let the potatoes steam dry for 1 to 2 minutes, then shake the pan to scuff the potatoes up a bit.
While potatoes are boiling, preheat the oven to 220 C / Gas 7, put the butter on a baking tray and put in the oven for a couple of minutes to melt butter, so it coats the bottom of the tray. Place the potatoes on the tray, drizzle a little olive over them all and season with salt and pepper.
Roast in the oven for 30 minutes.
Strip the leaves off the rosemary sprigs and put them in a small bowl with about 60ml to 80ml olive oil. Take potatoes out the oven and with a potato masher press down on each roaster enough so the skin splits on the sides, then drizzle the oil and rosemary over all the potatoes and return to the oven for 10 to 15 minutes.
Finally, remove roasters from the oven, give them a good dusting with the ground garlic granules and return to the oven for a final 5 minutes.