In a large pot, cover potatoes with water and season with salt. Bring to a boil and cook until totally soft, 15 to 18 minutes. Drain and return to pot. Use a potato masher to mash potatoes until smooth. Meanwhile, in a small saucepan over medium heat, heat oil and rosemary then add garlic and cook until fragrant, 1 minute. Discard rosemary and pour oil over potatoes. Add butter and stir until completely combined and creamy. Season with salt and pepper. Transfer potatoes to a serving bowl and season with more pepper and garnish with rosemary and parsley.