Heat the oven: Heat the oven to 425°F and butter a 10-inch cast-iron skillet.
Whisk the dry ingredients: Whisk the flour, sugar, and cinnamon in a large bowl.
Cut in the butter: Cut in the cold butter with a pastry blender or two forks until pea-sized pieces remain.
Stir in the wet ingredients: Stir the mashed sweet potato and buttermilk together, then stir into the flour mixture just until the dough holds together, adding a splash more buttermilk only if it will not come together.
Pat and cut: Turn the dough onto a lightly floured surface, pat to 1 inch thick, and cut with a floured 2-1/2-inch cutter, pressing straight down without twisting.
Bake: Set the biscuits touching in the skillet and bake 15 to 20 minutes, until tall and golden.
Make the maple butter: Beat the softened butter, maple syrup, and salt until creamy, and serve with the warm biscuits.
