This is the image with the black lining under my icing:
I rendered this with Cycles. What have I done wrong? I used the Solidify modifier on my icing and a Subsurf modifier.
As far as I can see, that's just your lighting. Maybe post the .blend on a site like Blend-Exchange? All I can see in the picture is a shadow made by a the top half being a little bigger than the bottom. :/