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 potatoes to pot.
Use a potato masher to mash potatoes until smooth.
Meanwhile, in a small saucepan, melt butter and milk until warm.
Pour over warm milk-butter mixture and stir until completely combined and creamy. Add sour cream and stir until combined.
Season mashed potatoes generously with salt and pepper.
Transfer potatoes to a serving bowl and top with remaining two tablespoons butter. Season with more pepper before serving.