Peel the yukon gold potatoes, quarter them lengthwise, then cut crosswise into 3/4-inch pieces. Place in a large saucepan with 5 cups cold water and 1 tsp. salt. Bring to a simmer over medium heat, then continue simmering until tender, about 15 minutes. Using a slotted spoon, transfer the yukon golds to a ricer or food mill and puree into a bowl; reserve the saucepan of water. Cover the yukon gold puree and keep warm in a low-temperature oven.
Peel the sweet potatoes and cut them into 1/2-inch cubes. Add to the reserved pan of water and simmer over medium heat until tender, about 15 minutes. Drain and puree through the ricer into the bowl with the yukon golds. Stir well to combine, adding the buttermilk, 1/2 tsp. salt and pepper to taste.
Rinse and wipe out the saucepan. Add the EVOO and scallion greens and cook over medium heat until heated through, 1 to 2 minutes. Spoon the warm scallion oil over the potato mixture.