Clean and scrub the potatoes. Remove any bad spots. Peel and cut into even sized chunks.
Put the potatoes and salt in a large pot and cover with cold water. Bring the pot to a boil and cook the potatoes until they are tender, about 15-20 minutes - they should be tender enough that a fork can easily stab through them.
Drain the water from the pot. With a potato masher, mash the potatoes. Add the warmed butter, mayonnaise, and milk into the potatoes and continue to stir until smooth.
Season the mashed potatoes with salt, black pepper, garlic powder, and fresh parsley. Check flavor and add more seasoning, if needed. If the masked potatoes are too thick, add more milk, in small increments, until the potatoes have reached your desired texture.