Preheat the oven to 200 degrees Celsius/ 400 degrees Fahrenheit. Poke the sweet potato with a fork a few times on each side. Bake in the preheated oven until tender, about 40 – 50 minutes. Leave until cool enough to handle, peel, mash well and let cool. Weigh the needed amount and set it aside.

Reduce the oven temperature to 180 degrees Celsius/ 350 degrees Fahrenheit. Grease a baking tin of about 20 cm/ 8 inch and line the bottom with parchment paper. Leave the parchment paper hang over 2 walls of the pan, this way it will be easier to lift the brownie out of the pan.

Melt the butter in a small saucepan. Set aside until you prepare the rest. Mix together the flour, unsweetened cocoa powder, baking powder, and salt. Set aside.

In another bowl mix together the melted and cooled butter, sweet potato puree, sugar, and egg. Carefully fold the flour mixture into the wet ingredients. Stir until just combined.

Fold in the chopped walnuts (and some extra chocolate chips, if you desire). Pour the batter into the prepared tin. Sprinkle with chocolate chips.

Bake for about 17-20 minutes, depending on your oven. A toothpick inserted in the middle of the brownie should still have a few crumbs hanging to it, but it should not be wet.