Preheat the oven to 350F; line an 8 by 8-inch baking dish with 2 pieces of parchment paper so it hangs over all 4 sides (so you can easily lift the brownies out later).
Add the sweet potato cubes to a medium saucepan and cover by 2 inches with cold water. Cover the saucepan and bring to a boil, and then turn the heat down and simmer until the potato is very soft, about 10 to 12 minutes. Drain well and cool slightly.
Add the sweet potato, coconut oil, and coconut sugar to a blender or food processor and process until pureed, stopping to scrape down the sides as necessary. Add the eggs, vanilla, and dissolved espresso and pulse to combine. Add the cocoa powder, flaxseed meal, salt, and baking soda, and process until smooth, scraping down the sides as necessary.
Pour the batter into the prepared dish, let the batter sit for 5 minutes (it will thicken), and then sprinkle the chocolate chips on top. Bake until the brownies are set along the outside and feel dry to the touch, but a wooden pick inserted in the center comes out wet, about 28 to 30 minutes (be careful not to over-bake!).
Cool completely before cutting. Store any leftovers wrapped in the fridge for up to 3 days.