Pre-head your oven to 430°F (220°C/200°C fan forced),
Put your sweet potatoes on a tray. Bake for fifty minutes in the oven until soft and cooked through.
Allow to cool, peel of skins and puree in a food processor until smooth. You should have one cup or 250gms sweet potato puree.
Reduce your oven temp to 355°F (180°C/160°C fan forced).
Grease and line the brownie tin.
Chop the chocolate into small pieces.
Add the chocolate and butter to a microwave proof bowl. Microwave for a few minutes until butter and chocolate are melted. Whisk well.
Add the vanilla to the sweet potato puree and whisk together.
Add the sweet potato to the chocolate and butter and mix well.
Put a sieve on top of the bowl and add the flour, coco powder, salt and baking power and mix well.
Tip into the lined brownie tin and smooth down. Sprinkle the chocolate chips over the top.
Bake for forty five minutes or until just cooked. A toothpick inserted into the brownie should come out with crumbs on it but not uncooked mix or be to dry.
Allow to cool completely before cutting into squares or rectangles and serve.