Preheat the oven to 350°F (180°C). Line an 8-inch square (20cm) baking pan with parchment and coat with cooking spray.
In a small bowl, combine the flaxseed with 6 tablespoons (89ml) water. Set aside for 10 minutes.
Meanwhile, steam the sweet potato in a steamer basket on the stove until very tender, 8 minutes. Alternatively, put the sweet potatoes in a microwave-safe bowl with ½ cup water, cover, and microwave on high until very tender, about 4 ½ minutes; drain.
Combine the sweet potato, flax mixture, almond butter, coconut oil, maple syrup and vanilla in a food processor. Process until completely smooth, stopping once to scrape down sides of the bowl, 1 minute.
Sift the cocoa powder, baking soda, and salt over the sweet potato mixture; pulse until completely blended.
Transfer the batter into the prepared pan. Sprinkle with the chocolate chips.
Bake until a skewer inserted in the center comes out with just a few moist crumbs on it, about 28 minutes.
Cool in the pan on a rack for at least 30 minutes. Cut into 16 squares and serve.
Keep leftovers in an airtight container in the refrigerator for up to five days.
