Peel potatoes and boil until fully cooked and flaky.
Drain potatoes and mash. Run the mash through a food processor or best with a hand mixer.
In a bowl, combine sour cream, cream cheese, ¾ of the butter, milk, and onion salt.
Mix potatoes with the combined ingredients.
Preheat the oven to 400°.
Put the mixture into a casserole dish, top with remaining butter, and bake for 30 minutes.