1. In a medium saucepan, cook the whole sweet potato in boiling (unsalted) water until tender when pierced with the tip of a sharp knife, about 20 minutes. Let the sweet potato cool, then pare it and mash until smooth. You should have about 1 cup mashed sweet potato. Let the cooking water cool to warm (105° or less).

2. Position a rack in the center of the oven and preheat to 425°.

3. In a medium saucepan, stir the mashed sweet potato with the melted butter until smooth. Transfer the mixture to a medium bowl. Stir in the milk, sugar, and egg. Sift the cake and all-purpose flours, baking powder, and salt into a medium bowl, and then stir into the liquids to combine. Knead briefly in the bowl to form a soft dough.

4. On a floured work surface, roll out the dough to ¾-inch thickness. Using a 2½-inch round cookie cutter, cut out biscuits. Gather up the scraps, reroll, and repeat the procedure until 12 biscuits are cut out. Transfer the biscuits to an ungreased baking sheet.

5. Bake the biscuits until golden brown, 15 to 20 minutes. Serve hot, warm, or at room temperature.

MASHING SWEET POTATOES

When cooking sweet potatoes (also called Louisiana yams) to mash and use in baked goods, boil them whole and unpeeled. While it takes more time, the potatoes won't become waterlogged. Mashed sweet potatoes that are too wet can throw off the flour measurement in your dough, ruining the bread. For each cup of mashed sweet potatoes, allow 1 medium raw sweet potato, weighing about 9 ounces. For the smoothest mashed sweet potatoes, rub pared cooked sweet potatoes through a fine-mesh sieve or put them through a ricer. If you like little chunks of mashed taters in your breads (and they do add a certain character), use a masher or even an electric mixer. But do not put them in a food processor, which will turn them into a gluey mess.

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