Preheat oven to 450°.
Line a baking sheet with parchment paper.
To make the sweet potato purée, pulse cooked or canned sweet potato in a food processor until smooth. I used canned sweet potato and it worked well.
In a small bowl, whisk the sweet potato purée with the buttermilk. If you don't have buttermilk, use regular milk, subtracting 1 TBS, and adding 1 TBS white vinegar in its place. Let it sit on the counter for a few minutes before using.
Meanwhile, in a medium bowl whisk the flour with the brown sugar, baking powder, baking soda, salt and pepper. Sprinkle the grated butter over the dry ingredients.
Using a pastry blender cut the butter into the flour until the mixture resembles very coarse crumbs, with some of the butter about the size of peas. Stir in the sweet potato mixture until a soft dough forms. Do not overwork the dough.
Turn the dough out onto a floured work surface and pat it into a 1-inch-thick round. Using a 2-inch biscuit cutter, cut out as many biscuits as you can. I got six. Carefully press the scraps together and cut out more biscuits.
Arrange the biscuits on the prepared baking sheet and bake for about 14 minutes, until golden brown. Brush with the melted butter and serve warm.