Sour Cream Mashed Potatoes

3 lb russet potatoes, peeled, cut into 2″ pieces
1 lb Yukon gold potatoes, peeled, cut into 2″ pieces
kosher salt
1 cup heavy cream
1 cup whole milk
½ cup (1 stick) unsalted butter, plus more for serving
Freshly ground black pepper
½ cup sour cream

Place potatoes in a large pot and cover with cold water by 2″. Generously season with salt and bring to a boil. Reduce heat and simmer until tender, 10-15 minutes. Drain and transfer potatoes to a baking sheet; let stand until dry, 10-15 minutes. Set pot aside

Meanwhile heat cream, milk, and butter in a medium saucepan over medium heat; season with salt and pepper. Pass potatoes through a ricer into reserved pot. Mix in warm milk mixture, then sour cream; season with salt. Serve topped with pepper and more butter, if desired.