Preheat oven to 425°F. Wash and poke the sweet potatoes all over with a fork.
Roast them on a foil-lined sheet pan for an hour, until fork tender.
Remove the sweet potatoes and let them cool enough to be handled. Remove the flesh and mash thoroughly.You need 2 cups of puree for the brownies, and 1 cup for the frosting. In a large bowl, stir together the sweet potato puree, melted coconut oil, vanilla extract, eggs, maple syrup, and cocoa powder until combined.
In a separate bowl, mix the flour, baking powder, and salt. Pour the dry ingredients into the wet bowl, a little at a time, and mix.
If using nuts, stir in the chopped pecans.
Preheat your oven to 350°F. Spray a baking pan with non-stick cooking spray and line the bottom with parchment paper.
Pour the brownie mixture into your baking pan and bake for about 25 minutes on the center rack.
Let it cool completely in the refrigerator.
Meanwhile, stir together the remaining 1 cup of sweet potato puree with the melted chocolate.
Cover your cooled brownies with the sweet potato frosting. Put the brownies back into the fridge until the frosting is set. Enjoy!