Make the Scones: Preheat oven to 400 degrees F. Line a baking sheet with parchment paper.
Prick the sweet potatoes all over with the tines of a fork and bake on the prepared pan until tender, 30 to 40 minutes. Remove from the oven and let cool to the touch, about 10 minutes.
Cut the potatoes in half lengthwise, scoop out the flesh into a medium bowl, and lightly mash with a fork. Cover the bowl and refrigerate until cool, at least 20 minutes.
Increase the oven temperature to 425 degrees F. Line two baking sheets with parchment paper; set aside.
In a large bowl, whisk together the flour, sugar, baking powder, salt, cinnamon and nutmeg. Use your fingers or a pastry blender to incorporate the butter into the flour mixture until the dough is crumbly and contains lumps the size of small peas.
Add the cooled sweet potatoes and ¼ cup of the heavy cream and lightly stir into the flour mixture. Add the remaining ¼ cup cream and the buttermilk and stir until just mixed. Do not overwork the dough.
Turn the dough out onto a lightly floured surface and gently knead it a few times until it comes together into a cohesive mass. Lightly coat your hands with flour and gently pat the dough into ¾-inch-thick rectangle (mine was about 11x9 inches in size). Cut the dough into 6 squares, then cut each square on the diagonal to make triangle-shaped scones.
Transfer the scones to the prepared baking sheets and bake for 15 to 20 minutes, or until the scones are a light golden brown. Immediately transfer the scones to a wire rack.
Make the Glaze: In a medium bowl, whisk together the powdered sugar, maple syrup, heavy cream and salt, until smooth. Add more cream a tablespoon at a time if needed to thin the icing to a drizzling consistency. Drizzle the icing over the warm scones. Serve the scones warm or at room temperature. The scones are best eaten the same day they are made, but can be stored, wrapped in plastic wrap, at room temperature for up to 2 days.