Heat oven to 400 F. Lightly coat a large baking dish with cooking spray.
In a large mixing bowl, add the whole potatoes, garlic cloves, olive oil, rosemary, salt and pepper. Use your hands to mix until the potatoes are coated with the oil and spices.
Arrange the potatoes in a single layer in the prepared baking dish. Cover with a lid or aluminum foil and bake for 25 minutes.
Remove the lid or foil. Turn potatoes and bake uncovered until the potatoes are soft and slightly browned, about 25 minutes.
Transfer to a serving bowl and mix with butter. Sprinkle with parsley and serve.