Place the potatoes in a large pot with 1 tablespoon of kosher salt (not table salt) and enough cold water to cover the potatoes by 1-inch.

Bring to a boil, then reduce heat to a gentle simmer until the potatoes are just fork tender, about 10 minutes. Drain the potatoes in a colander and return them to the pot.

Add 4 tablespoons of diced butter; stir until melted and combined. Then add in the sour cream, milk, mustard, remaining 1/2 tablespoon salt, and the pepper; mash with a potato masher until the desired consistency is reached. (Alternatively, you can use an electric mixer for extra smooth and silky mashed potatoes, but be careful not to over-mix or you could end up with gummy potatoes!)

Transfer hot mashed potatoes to a serving bowl and top with remaining two slabs butter and finely chopped parsley.