Preheat the oven to 160C/325F
Place sweet potato in blender and mix until a puree forms.
Add all other ingredients and mix until blended and smooth.
Pour the pudding mixture over the cooled chocolate biscuit base.
Place the nutella in a small saucepan and warm over a low heat until it softens.
Evenly pour the softened nutella over the pudding.
Make small discs (about the size of your thumb nail) with the cookie dough and place evenly over the nutella.
Place the tart in the oven and bake for 30-34minutes, or until the cookie dough is slightly browned and the pudding has set.
Allow the tart to cool before serving.
Cover and store in the fridge...maybe hide it right at the back so one else finds it and you can eat it all yourself.