Preheat oven to 400°. Make mashed potatoes: In a large pot, cover potatoes with water and add a generous pinch of salt. Bring to a boil and cook until totally soft, 16 to 18 minutes. Drain and return to pot. Use a potato masher to mash potatoes until smooth. Add melted butter, milk, and sour cream. Mash together until fully incorporated, then season with salt and pepper. Set aside.