Put the potatoes in a medium pot and cover with cold water by 1 inch. Add a pinch salt and bring to a simmer over medium-high heat.
Simmer until very tender, 12 to 15 minutes. A fork should easily piece through the whole chunk. Drain and return to the pot. Add the yogurt, olive oil, cayenne, 1/2 teaspoon salt and 1/4 cup water. Mash well, using a potato masher, until smooth and creamy. Add additional water if needed to achieve the desired consistency. Stir in the scallions and adjust the seasoning to taste with additional salt and cayenne.
Recipe courtesy Melissa Gaman for Cooking Channel