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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 until mixture resembles coarse meal, with some pea-sized lumps of butter remaining. In a small bowl, whisk together sweet potato puree and buttermilk; stir quickly into flour mixture until combined (do not overmix).
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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 (do not reuse scraps more than once).
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Bake the biscuits: Preheat oven to 425 degrees, with rack on lower shelf. Butter an 8-inch cake pan. Arrange biscuits snugly in pan (to help them stay upright). Brush with melted butter. Bake until golden, rotating once, 20 to 24 minutes.