In this recipe, Yukon Gold potatoes and sweet potatoes are roughly mashed together with leeks for a beautiful mottled white-and-orange take on mashed potatoes. The creme fraiche topping offers a tangy counterpoint. 2-Toned Mashed Potatoes Active Time: 20 minutes Total Time: 35 minutes Serves 12 1 tablespoon buttermilk plus 2/3 cup, divided 2 tablespoons creme fraiche or sour cream 1/2 teaspoon onion powder or granulated onion Pinch of salt plus 3/4 teaspoon, divided 1 1/2 pounds Yukon Gold potatoes, cut into 1-inch chunks 1 1/2 pounds sweet potatoes, peeled and cut into 1-inch chunks 1 small leek, halved lengthwise and thinly sliced, white and light green parts only 2 tablespoons butter, cut into 4 pieces 1/4 teaspoon freshly ground pepper 2 tablespoons minced fresh chives Bring a large pot of water to a boil. Combine 1 tablespoon buttermilk, the creme fraiche (or sour cream), onion powder (or granulated onion), and …