Position a rack in the center of the oven and pre-heat the oven to 400°F.
Line a baking sheet with parchment paper and set aside.
In a large bowl whisk together the flour, baking powder, baking soda, salt, pepper and nutmeg. Add the Parmesan and rosemary and whisk again to incorporate the ingredients.
Add in the cubed butter and using a pastry blender or your fingers blend the butter into the flour until the mixture looks like coarse meal.
In a small bowl combine the buttermilk with the cooked and mashed sweet potato, whisk to blend and then add it to the flour mixture.
Mix the ingredients until they just come together, do not over-mix the dough or your biscuits will be tough.
Turn the dough out onto a lightly floured surface, knead it a few times and then pat into a circle about 1-1/2 inch high.
Using a round, 2-inch wide cutter or glass (edge dipped in flour before cutting into dough), cut as many biscuits as you can. Gather the scraps, reshape into a disk and cut again.
Place the biscuits on the baking sheet. Once all the biscuits have been cut and placed onto the baking sheet, whisk the egg yolk and cream (or milk) in a small bowl and brush the tops (only the tops) of the biscuits.
Bake in the oven for 16 to 18 minutes or until the biscuits are golden brown. Let cool for a few minutes and then serve while still warm with butter.
Notes
This biscuit recipe uses a previously cooked large sweet potato. If you are starting from scratch, remember to start by cooking the sweet potato. You'll need about one large sweet potato or about one cup of cubed sweet potato to have enough for this recipe.
Baked sweet potato biscuits can be frozen in a freezer bag for up to 3 months. Defrost biscuits in the refrigerator overnight and warm them in the oven before serving.