Peel and dice the potatoes into small chunks and add to a large pot and cover with water. Bring the water to a boil and reduce it to a simmer. Cook for 15 to 18 min or until a knife can cut through a piece of potato easily. Drain the potatoes and place them back in the pot and mash the potatoes with a potato masher. When most of the potatoes are broken up, add the butter, milk, parmesan cheese, garlic, and pepper. Continue mashing the potatoes until they are smooth.