Use a fork to prick the skin of the sweet potato several times. Place the sweet potato on a microwave safe plate, then microwave on high for 5 minutes. After five minutes, use the fork to pierce the sweet potato in the thickest part. If it's not very soft all the way through, microwave for an additional 1-2 minutes, or until the center is very soft.
Carefully remove the sweet potato from the microwave (the plate will be VERY hot). Use a towel to grasp the sweet potato as you cut open the skin and scoop out the flesh. Scoop out one packed cup of sweet potato and place it in a medium bowl. Use the fork to mash the sweet potato until smooth. Set it aside to cool further.
Preheat the oven to 400ºF. In a separate large bowl, combine 2 cups of the all-purpose flour (reserving the last 1/4 cup for later), baking powder, sugar, salt, and nutmeg. Stir until these ingredients are very well combined.
Use a cheese grater to grate the frozen butter into the bowl with the flour mixture. Take care to guard your fingers and knuckles when you get down to the end of the stick of butter. Stir the grated butter into the flour mixture until they are evenly combined.
Add the milk to the bowl with the mashed sweet potato. Stir or whisk the sweet potato and milk together until they are well combined. A few small chunks of sweet potato are okay.
Finally, pour the sweet potato and milk mixture into the bowl of flour and butter. Stir to combine the wet and dry ingredients until one cohesive ball of dough forms. Because sweet potatoes can vary in moisture content, you may need to add extra flour or milk to the dough to get the right consistency. The biscuit dough should be slightly sticky, but not so wet that it is glossy, and no dry flour should remain on the bottom of the bowl. If the dough is too dry (not forming a single ball of dough) add a couple tablespoons of milk. If it's too wet, dust it liberally with the reserved flour.
Turn the dough out onto a well floured surface. Press it down into a flat circle, about 1-inch thick. Use a biscuit cutter or a drinking glass to cut biscuits from the dough. Gather up the dough scraps, reshape them into a 1-inch thick circle, and cut more biscuits. Repeat this until no more dough remains. Place the cut biscuits on a parchment lined baking sheet.
Bake the biscuits for about 20 minutes, or until they are golden brown on top. Serve warm, with butter or maple syrup.