Preheat the oven to 400 degrees . Line a large baking sheet with parchment. In a large bowl, whisk together the flour, brown sugar, baking powder, baking soda, salt and 1/4 tsp. pumpkin pie spice. Add the chilled butter and work into the dry mixture with your fingertips until finely blended.
In a medium bowl, whisk together the mashed sweet potato and milk. Pour the sweet potato mixture into the dry mixture and stir with a fork until the dough just comes together.
Turn out the dough onto a floured work surface; pat into an 8-inch round. Using a 2-inch cutter, cut out 8 biscuits; transfer to the prepared baking sheet. Gather the scraps, pat out into a 5-inch round and cut out 6 more biscuits; place on the baking sheet.
Brush the biscuits with the melted butter. Mix the granulated sugar with the remaining 1/8 tsp. pumpkin pie spice; sprinkle on the biscuits. Bake until puffed and golden, 18 minutes. Let cool for 10 minutes before serving.