Preheat oven to 400°F. Line a baking sheet with parchment paper.

Sift together flour, sugar, baking powder, salt, nutmeg, and 1/2 teaspoon of the cardamom. Add butter; cut into the flour mixture with a pastry blender or a fork until butter pieces are the size of corn kernels. Stir in ginger and zest. Stir together sweet potatoes, buttermilk, and 1/4 cup of the cream in a small bowl. Make a well in the center of the flour mixture; place sweet potato mixture in well. Working quickly, stir until dough just comes together.

Sprinkle prepared baking sheet liberally with flour. Place dough on parchment, and divide dough in half. With floured hands, shape each dough half into a circle between 6 and 7 inches in diameter and about 1 inch thick. Brush circles evenly with remaining 2 tablespoons cream. Stir together turbinado sugar and remaining 1/8 teaspoon cardamom in a small bowl, and sprinkle evenly over dough circles. Run a bread knife or bench scraper under cold water, and cut each circle into 8 wedges (like cutting a pie). Carefully pull wedges away from the center to separate them by about 3/4 inch. (Don’t worry about any excess flour on your parchment paper.)

Bake on middle rack in preheated oven until golden brown and almost cooked through, about 30 minutes. Transfer baking sheet to lowest oven rack; bake until bottoms of scones are fully cooked, about 5 minutes. Serve warm or at room temperature.