In a large bowl, whisk together 1 3/4 cups all-purpose flour, 2 Tbs light-brown sugar, 2 1/2 tsp baking powder, 1 tsp salt, and 1/2 tsp baking soda.
With a pastry blender or two knives (or not-too-warm fingers), cut in 6 Tbs chilled unsalted butter, cut into pieces, until mixture resembles coarse meal, with some pea-size lumps of butter remaining.
In a small bowl, whisk together 3/4 cup chilled sweet potato purée and 1/3 cup buttermilk; stir quickly into flour mixture until combined (do not overmix).