Preheat the oven to 325℉. Line an 8x8 or 9x9 square pan with parchment paper or spray with cooking spray.
Prick the sweet potato multiple times and microwave it for 5 minutes. Carefully cut the sweet potato in half, and check for doneness with a knife. If the sweet potato is tender, set it aside. If it's still firm, microwave for additional 1-2 minutes or until tender. Set it aside to cool.
In a medium bowl, combine the gluten free flour, cocoa powder, salt, espresso powder and baking powder. Stir until evenly combined and there are no visible lumps of flour. Set aside.
Place the butter in a large microwave safe bowl and microwave for 45 to 60 seconds, or just until melted.
Add half of the chopped chocolate to the melted butter and stir until the chocolate is completely melted.
Add the sugar to the melted butter and chocolate and stir until combined.
Once the sweet potato is cool enough to handle, mash it with a fork. Measure ½ cup of mashed sweet potato and add it to the butter mixture. Stir until combined.
Add the eggs and vanilla to the wet mixture and stir until combined.
Add the dry ingredients and stir just until combined. Do not overmix.
Fold in the remaining chopped chocolate.
Pour the batter into the prepared baking pan. Bake for 35 to 40 minutes, checking at 35 minutes. A toothpick inserted into the center should come out with a few moist crumbs but should not be coated with wet batter.
Let the brownies cool completely before cutting.
Optional: sprinkle with flaky salt or cocoa powder before serving.