Preheat the oven to 400ºF.

Scrub the potatoes clean and pierce with a fork. Line a baking sheet with foil and place the potatoes on the baking sheet. Bake the potatoes until they are tender, about 45 minutes.

Line another baking sheet with parchment paper or a silpat liner. Set aside.

In a food processor, combine the flour, sugar, baking powder, and salt. Pulse a few times to combine.

Add 5 tablespoons of the butter to the food processor. Pulse the mixture just until it resembles coarse crumbs.

Add the 1 cup of sweet potatoes and buttermilk to the food processor. Pulse just until the mixture comes together.

Lightly flour a counter and turn the dough out. Knead the dough a few times to bring it together.

Roll the dough into a rectangle about 1/2-inch thick. Fold one side of the dough over to by 1/3, then fold the other side over the first side. Flatten the dough into another rectangle (this one will perpendicular to the direction of the first rectangle that you made). Fold the dough into thirds again.

Roll the dough one last time to about 1/2-inch thick. Using a 2-inch cookie cutter, cut the biscuits, pressing straight down on the cutter, being careful not to twist or turn it. Place the biscuits on the prepared baking sheet. Re-roll the dough the same way you rolled it out the first time (folding into thirds twice) then cut the remaining dough into biscuits. Discard any dough that is left over.

Bake the biscuits in the 400ºF oven until they are lightly browned, about 15 minutes.

While the biscuits are baking, melt the remaining tablespoon of butter.

When the biscuits are done, remove from the oven and brush with the melted butter.