How to Make It
Preheat oven to 400°F. Line a baking sheet with parchment paper; set aside.
Whisk together all-purpose flour, hazelnut flour, granulated sugar, baking powder, and salt in a large bowl. Using a pastry blender or 2 knives, cut butter into flour mixture with until mixture resembles corn kernels. Stir together mashed sweet potatoes, buttermilk, and 1/4 cup of the cream in a small bowl. Make a well in center of flour mixture; working quickly, add sweet potato mixture, and stir just until dough comes together.
Sprinkle parchment-lined baking sheet liberally with flour. Divide dough in half. Place 1 dough half on prepared baking sheet; cover and chill remaining dough half. With floured hands, shape dough into an 8-inch circle (about 1 inch thick). Brush evenly with 1 tablespoon of the cream. Sprinkle evenly with 1 tablespoon of the turbinado sugar. Using a floured bench scraper or knife, cut dough into 8 wedges (like cutting a pizza). Carefully pull scones away from center to separate them by about 3/4 inch.
Bake in preheated oven until golden brown and cooked through, 25 to 30 minutes. Cool 5 minutes; remove scones from baking sheet. Repeat with remaining dough half, cream, and turbinado sugar.
Place hazelnut-chocolate spread in small microwavable bowl. Microwave on HIGH until runny, about 20 seconds. Spoon into a small ziplock plastic bag, and snip a small hole in 1 corner. Drizzle decoratively over scones. Sprinkle with hazelnuts, if desired. Serve warm or at room temperature.
Note: Scones can be cut in advance, and frozen on a baking sheet. Once they are completely frozen, transfer scones to a ziplock plastic freezer bag for longer storage. Bake directly from frozen. You may need to add a couple of minutes to the baking time.