How to Make It
In a mixing bowl, stir together flour, baking powder, sugar, and salt until well combined. Using a pastry cutter or your hands, cut butter into flour until only a few pea-size pieces remain. In a separate bowl, stir together buttermilk and sweet potato until homogenous. Add to dry ingredients and use a spoon to mix ingredients together until a shaggy dough forms. Flour your hands and press the dough together until all of the dry ingredients in the bowl are incorporated into 1 large dough mass.
Line a 9-inch round metal cake pan with parchment paper and grease with cooking spray. Transfer dough to a well-floured surface. Press the dough down and use a rolling pin to roll the dough out to a 3/4-inch thick disc. Using a 3-inch round cutter, cut out 1 biscuit from the dough and place it in the center of the prepared cake pan. Cut out 4 more biscuit rounds from the dough and place them around the perimeter of the cake pan, allowing them to touch each other and the center biscuit. Reroll the dough scraps to 3/4-inch thickness, and cut out remaining 2 biscuits. Fit them snuggly in the cake pan so all of the biscuits are touching (6 biscuits should be surrounding the biscuit in the center). Transfer to the freezer for at least 1 hour.
Preheat oven to 350°F. Whisk together the egg yolk and water. Brush the surface of each biscuit evenly with egg wash. Bake until the tops have become golden brown and the biscuits have baked all the way through, 40 to 45 minutes.
To make the mashed sweet potato, wrap sweet potatoes in aluminum foil and roast in a 425°F oven for 1 1/2 hours. Turn off the oven and allow the sweet potatoes to cool in the oven for 2 hours. This allows more of the starches in the sweet potato to transform into complex sugars, making the sweet potato flesh both sweeter and softer. Peel and mash the sweet potatoes.