Pre-heat oven on 180 C (356 F).
Line a baking tray roughly 12" by 9" with either tin foil or parchment paper.
Add all wet ingredients, including sweet potatoes, to a large mixing bowl and blend together thoroughly.
Add all dry ingredients to another bowl and mix together.
Now fold in your dry ingredients (including dark chocolate if using) and mix together gently. Don't over mix as you'll make your batter too tight, just gently mix until combined.
Pour into your baking tin and make sure the mixture is spread evenly.
Bake for around 22-23 minutes. Go for the shorter time if you want them more fudgy.
Cool and refrigerate for up to 10 days.
