Make the dough: In a large bowl, whisk together flour, sugar, baking powder, salt, and baking soda. With a pastry blender or two knives, cut in butter and shortening until mixture resembles coarse meal, with some pea-sized lumps.

In a small bowl, whisk together sweet potato puree and buttermilk; stir quickly into flour mixture until combined (do not overmix).

Shape the biscuits: Turn out dough onto a lightly floured surface, and knead very gently until dough comes together but is still slightly lumpy, five or six times. (If dough is too sticky, work in up to 1/4 cup additional flour.) Shape into a disk, and pat to an even 1-inch thickness. With a floured 2-inch biscuit cutter, cut out biscuits as close together as possible. Gather together scraps, and repeat to cut out more biscuits (try not to reuse scraps more than once or twice - dough can become to tough when overworked).

Bake the biscuits: Preheat oven to 425 degrees, with rack on lower shelf. Butter an 8-inch pan. Arrange biscuits snugly in pan (to help them stay upright). Brush with melted butter. Bake until golden, rotating once, 20 to 24 minutes.

Makes approximately 6 biscuits.