4 pounds russet potatoes (about 6 large potatoes)
Kosher salt and freshly ground black pepper
1 1/2 sticks (12 tablespoons) unsalted butter
1 1/4 cups heavy cream or half-and-half (see Cook's Note)
If you plan to freeze the potatoes, you will only need 3/4 cup cream total since no additional cream is needed to reheat. For extra-smooth potatoes, use a potato ricer or a food mill instead of a potato masher.