Make sure the potato is well mashed, with no lumps; this is best achieved with a potato masher or putting the potato through a ricer or food mill. Cream together the brown sugar and the mashed sweet potato and set aside.
In a large bowl, combine the salt, ice cold butter cubes, flour, baking powder, baking soda and cayenne pepper. Using your hands or a pastry knife, cut the butter into the flour until the flour looks like large grains of wet sand.
Blend the potato and buttermilk into the flour mixture, stirring only until just combined. The dough will be very sticky. Wrap it in cling film and chill for 20 minutes.
Generously flour your work surface and a rolling pin and roll the dough out until it is about ½ inch thick. Using a knife, an upturned glass or a biscuit cutter, cut out your desired shapes and place them on a greased baking sheet.
Melt the remaining butter and brush it onto the tops of the biscuits. Sprinkle on coarse sea salt and freshly ground pepper to each biscuit if desired.Bake at 350*F for 12 minutes, then rotate the pan and bake another 12 minutes more. Remove from the oven and let cool slightly before serving.