Put the sweet potato chunks into a large pot and cover with cold water by 2-inches. Stir in 1 tsp. of the salt. Bring to a boil over hight heat. Reduce the heat and simmer until the potatoes are tender when pierced with a fork, 15 to 20 minutes. Drain thoroughly, return to the pot and mash until smooth. You will need 1 ½ cups mashed sweet potatoes. Let cool.

Transfer the cooled mashed potatoes to a medium bowl. Stir in the brown sugar and buttermilk until blended.

In a large bowl, whisk together the flour, baking powder and ½ tsp. salt. Using the large holes on a box grater set on a sheet of parchment or waxed paper, grate the butter and finely chop the remaining nub. Add the butter to the flour mixture, using your fingers to rub and distribute it and breaking up any large clumps.  Add the sweet potato mixture and mix just until evenly blended.

Turn the dough out onto a floured board and pat into a 1-inch thick rectangle. Fold one side into the center, then the other side. Turn the dough horizontally. Gently flatten into a 1- inch thick rectangle again. Repeat the folding and turning 3 to 4 more times.

Preheat the oven to 425°F. Line a rimmed baking sheet with parchment paper.

Using a 2 ½-inch round biscuit cutter, cut the dough into 10 to 12 rounds. Re-roll scraps until all the dough is used. You should have about 12 biscuits. Arrange the biscuits on the prepared baking sheet, spacing them evenly. Brush the tops lightly with the melted butter.

Bake until lightly golden, 20 to 25 minutes. Cool slightly before serving.

Meanwhile, make the cinnamon butter.  In a small bowl, mix the butter, honey, cinnamon, and salt. Serve the butter with the warm biscuits.