Grease a 12 hole muffin pan, and preheat the oven to 180°C.
Boil the sweet potatoes in a saucepan of water for 12-15minutes until cooked through. Remove from the heat, allow to cool and then peel. Grate the potatoes on the fine part of the grater and set aside.
Roast the hazelnuts in the oven for 5-7 minutes. Remove and roughly chop.
In a bowl, beat the butter with the sugar until light and creamy. Add the eggs and beat until well combined.
Sift the flour into a bowl with the cocoa and baking powder. Add this to the creamed mixture alternating with the buttermilk. Stir in chocolate, hazelnuts and sweet potato.
Divide the mixture between a 6 hole muffin pan, and place in the oven.
Bake for 20-25 minutes until the muffins are risen and cooked through.
Allow to cool before drizzling over chocolate icing, or alternatively dust with cocoa and serve.
Make into a large cake (round or square) which keeps really well. Adjust your cooking times as necessary until the cake is cooked through.
Reprinted with permission of Sasko.