Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper.
In a bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, nutmeg, and brown sugar.
Cut in cold butter using a pastry cutter (or your hands) until the mixture looks like coarse crumbs.
Stir in mashed sweet potatoes and buttermilk, mixing just until combined.
Turn the dough onto a floured surface and gently roll it ½ inch thick.
Use a biscuit cutter (or a glass) to cut out rounds.
Place biscuits on the prepared baking sheet, leaving space between them.
Bake for 15 minutes or until golden brown.
While they bake, whisk together powdered sugar, coffee, and vanilla to make the drizzle.
Let biscuits cool slightly, then drizzle with coffee glaze before devouring.