Prick 1 medium sweet potato and roast at 400 degrees F for 45 to 55 minutes, until a knife slides in with no resistance. Scoop the flesh out and mash it completely smooth.

Measure 1 cup of mash, spread it on a plate and refrigerate at least 1 hour, until fully cold. This can be done up to 4 days ahead.

Stir the yeast and sugar into the warm water and let it stand 5 to 10 minutes, until foamy and slightly domed.

Whisk the flour, baking powder, baking soda and salt in a large bowl for 30 seconds to break up any clumps.

Add the cold cubed butter and rub it in with your fingertips or a pastry blender until the largest pieces are the size of peas, leaving some flat shards visible.

Whisk the cold mashed sweet potato and cold buttermilk together until smooth, then pour it into the flour along with the foamy yeast. Stir with a fork just until no dry flour remains.

Turn the soft dough onto a floured counter and knead about 8 times, until it holds together.

Cover the dough and let it rise at room temperature for 1 hour, or refrigerate it for up to 5 days. It will puff and soften rather than double.

Pat or roll the dough to ¾ inch thick and cut rounds with a 2 ½ inch cutter, pressing straight down and lifting straight up without twisting. Gather and re roll the scraps once.

Arrange the biscuits on a baking sheet with their sides just touching and let them rest 20 to 30 minutes while the oven heats to 425 degrees F. Cold dough needs closer to 40 minutes.

Bake 14 to 16 minutes, until the tops are deep golden and the sides are set. Brush with the melted salted butter as soon as they come out.