Preheat the oven to 400 degrees F, then generously butter a baking sheet.
Wrap sweet potatoes in foil in a single package & bake until tender, about 1 hour, then remove from the oven, unwrap & set on a wire rack to cool somewhat. Raise oven temperature to 425 degrees F.
When potatoes are cool enough to handle, peel & mash them, so that you have 1 cup of puree.
In a mixing bowl, whisk together the flour, baking powder, brown sugar, nutmeg & salt, then stir in the pepper before adding the butter. Cut in the butter, combining & mixing until coarsely crumbled.
Make a well in the center & add the cup of mashed sweet potatoes, stirring together gently with a fork, then stir in enough cream to make a sticky but manageable dough.
Transfer the dough to a floured surface & pat or roll to a thickness of 3/4 inch. Use a floured 2 1/2-inch biscuit cutter to cut out rounds of the dough, & transfer them to the prepared baking sheet. Re-roll the dough as necessary, until the dough is used up.
Lightly brush the tops of the biscuits with the beaten egg, then bake until the biscuits are golden brown, about 15 minutes.