How to Make It
Preheat the oven to 400°. Pierce the sweet potatoes all over with a fork. Set the potatoes on a cookie sheet and bake for 1 hour, or until tender. Let cool slightly. Using a spoon, scoop the potato flesh from the skins into a bowl and mash with a fork; you should have 1 1/2 cups of mashed sweet potatoes. Refrigerate until chilled.
In a large bowl, mix the 2 1/2 cups of flour with the brown sugar, baking powder, salt and cayenne pepper. Cut in the butter with a pastry blender. In a small bowl, mix the sweet potatoes with the milk. Add to the dry ingredients and stir with a wooden spoon until all of the flour is incorporated; the dough will be quite sticky. Refrigerate the dough for at least 15 minutes.
Preheat the oven to 425°. On a well-floured work surface, pat out the dough until it is 1/2-inch thick. Using a floured 2-inch round or fluted cutter, stamp out as many biscuits as you can and transfer them to a parchment paper-lined baking sheet. Re-roll the scraps and stamp out more biscuits. Brush the tops of the biscuits with the egg wash and press a small herb sprig into each one. Bake the biscuits for about 20 minutes, or until golden. Serve warm or at room temperature.