preheat the oven to 325°. Grease a 9 inch springform cake pan with a little oil and line the bottom with nonstick parchment paper
steam the sweet potatoes over a saucepan of boiling water for 15 minutes until tender. Mash until smooth, then mix the orange zest and juice and let cool for 15 minutes.
Sift the flour, cocoa, and baking soda into a bowl. Cream the butter and sugar together in a large mixing bowl until light and fluffy. Gradually beat in the eggs alternately with the flour, until all have been added and the mixture is smooth. Stir in the mashed sweet potato and mix well.
Spoon into the prepared pan. Spread the top level and bake in preheated oven for 45 to 55 minutes, until well risen, the top is slightly cracked, and a skewer comes out cleanly when inserted into the center.
Let cool in the pan for 15 minutes, then loosen the edge and remove the pan and lining paper. Put on a cooling rack and cool completely.
To make the frosting, put the chocolate and cream in a bowl set over a pan of gently simmering water and heat for about five minutes, beating lightly from time to time until the chocolate is melted and the frosting is thick and glossy. Remove from the heat. Cool for 5 to 10 minutes, beating occasionally, until thick enough to hold its shape.
Spoon the frosting over the cake and spread with a knife. Decorate with grated orange zest, then put in a cool place for about an hour for the frosting to Harden.