1. Preheat oven to 450°. Stir together first 4 ingredients and 1/2 tsp. cinnamon in a large bowl; cut in butter with a pastry blender until mixture resembles small peas and dough is crumbly. Freeze 5 minutes.
2. Stir together sweet potatoes and 1/2 cup whipping cream. Add to flour mixture, stirring just until dry ingredients are moistened.
3. Turn dough out onto a lightly floured surface (mixture will be crumbly); knead 1 minute. Pat dough into a 3/4-inch-thick circle. Cut dough with a well-floured 2 1/2-inch round cutter, rerolling scraps as needed.
4. Line baking sheets with parchment paper; lightly grease paper with cooking spray. Place biscuits 2 inches apart on prepared baking sheets. Brush tops of biscuits with 2 Tbsp. whipping cream.
5. Stir together granulated sugar and remaining 1/4 tsp. cinnamon. Sprinkle dough rounds with sugar mixture.
6. Bake at 450° for 13 to 15 minutes or until golden brown.