Preheat the oven to 180ºC (350ºF). Line muffin tins with muffin cases.
Start by melting together the 70% dark chocolate and the butter. I find it easiest to do this by placing the chocolate and butter in a heatproof bowl set over a pan of simmering water. Ensure the water does not touch the bowl and stir occasionally until melted. Remove from the heat.
In a large bowl add the mashed sweet potato and pour over the melted chocolate and butter. Using a balloon whisk (or a wooden spoon) mix together well.
Add in the eggs, one at a time, mixing well after each one. Next incorporate the milk, sugar, vanilla and salt - the mixture should be smooth and runny.
Switch to a wooden spoon or spatula and gently fold in the flours, cocoa powder, baking powder, bicarb and chocolate chunks.
Divide the mixture between the holes of your muffin tin - when I made this recipe using regular muffin cases I got 15 muffins. Bake for about 20 minutes - they are ready when a skewer inserted into the centre of the muffin comes out clean.