Preheat oven to 350°F (177°C). Grease or line with parchment paper a 8-inch X 8-inch (20- X 20-cm) pan. (I used a pyrex pan.)
In a food processor, combine cooked sweet potatoes, coconut oil, eggs, sugar, and vanilla. (Make sure that the sweet potatoes have cooled sufficiently before adding eggs, so they don't get cooked on contact.) Process until the mixture is smooth and blended.
Add cocoa powder, flours, baking powder, and salt. Process until smooth, scraping down the sides if necessary.
Remove blade from food processor, and stir in ¾ cup walnut pieces by hand.
Spread the batter into a prepared pan. Bake for 35 minutes or until the centre tests done when poked with a fork. Set on a rack to cool.
After removing the brownies from oven and turning the heat off. Set the chocolate chips in an oven-proof container into the cooling oven. When they have melted just enough to stir smooth, stir in the almond butter.
Frost cooled brownies with prepared frosting. Sprinkle with additional walnut pieces, if desired.
A food processor is the perfect tool to make this recipe easily. Remove the blade before adding the nuts, so they don't get chopped too finely.
Allow the brownies to cool before applying the frosting.