Preheat oven to 350º F. Line an 8x8 baking pan with parchment paper, and set aside.
Start by combining the cocoa powder, coconut sugar, arrowroot, baking soda, and salt in a small bowl. Stir to combine the dry ingredients.
In a large mixing bowl, and the roasted sweet potato puree, egg, coconut oil, sunflower seed butter, vanilla and vinegar. The vinegar is optional but prevents the reaction between sunflower seed butter and baking which turns baked goods green.
Add the dry ingredients into the bowl with the wet ingredients, and stir to form a thicky, glossy, fudgy batter. It should be very thick.
Add the brownie batter to an 8×8 baking pan lined with parchment paper. Use wet hands or a wet silicone spatula to spread the batter into an even layer in the pan.
If desired, top the brownies with dark chocolate chunks and pecans. Keep them nut free by only using dark chocolate, or adding pumpkin seeds in place of the pecans.
Bake the brownies for 28 minutes. The long baking time will help remove some of the moisture in the sweet potatoes, yielding a chewy, fudgy brownie texture.
Gluten free brownies need to cool completely in the pan before slicing. For the cleanest cuts, allow the brownies to chill for 1-2 hours in the refrigerator prior to slicing.