How to Make It
Preheat the oven to 450°. Line a baking sheet with parchment paper. In a small bowl, whisk the sweet potato puree with the buttermilk. In a large bowl, whisk the flour with the brown sugar, baking powder, baking soda and salt. Sprinkle the cubed butter over the dry ingredients; using a pastry blender or two table knives, 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 just until a soft dough forms.
Turn the dough out onto a floured work surface and pat it into a 1-inch-thick round. Using a 2-inch round biscuit cutter, stamp out as many biscuits as you can. Gently press the scraps together and stamp out more biscuits. Arrange the biscuits on the prepared baking sheet and bake for about 15 minutes, until golden brown. Brush with the melted butter and serve warm.
To make the sweet potato puree: Pierce one 12-ounce sweet potato in a few spots with a fork and microwave at high power for about 10 minutes or roast at 350° for about 1 hour, until tender. Let the sweet potato cool slightly, then peel and puree in a food processor. Measure out 1 cup of puree and refrigerate until chilled.