Bake the sweet potato in the oven until soft (you'll notice when it's ready if the skin loosens). This can also be done a day ahead, as you want to be sure it's completely cooled.
Begin by preparing your flax eggs. For 1 flax egg, mix 1 tablespoon of flax meal with 3 tablespoons of water. You will need two flax eggs for this recipe. Let it sit, and after a few minutes, it will have an egg-like gelatinous consistency.
In a food processor, combine sweet potatoes, beans, cacao powder, coconut sugar, flax eggs, salt, sodium bicarbonate (or baking powder), and optionally vanilla. Blend until everything is well mixed.
Pour into a small, square, greased spring form and mix in the chocolate chips (or chocolate chunks). Bake at 350 degrees F / 175 C for 25 minutes. Cover with foil if the top starts to get dark.
Meanwhile, prepare the frosting by mixing the avocado with maple syrup (or agave), cacao powder, and, optionally, vanilla. I use a small food processor for this.
Once you remove the brownie from the oven, let it cool. It will be very soft once you take it out and firm up during the cooling process.
Add the avocado cream on top and slice into small squares. Enjoy!