Thank you for the info! I've always seemed to struggle with wrapping chocolate around a cake... just can't seem to judge if the consistency is ready for moving or not.
My solution is fun though and also eliminates the seam.
1. Use the same plastic as mentioned by Stephanie214.
2. Cut the strip of plastic to the desired height, perhaps taller than the cake by a cm or so.
3. Grease a cake tin that is a bit wide than the cake (unless you are doing to trim the edges of the cake, in which case use the original cake tin you baked it in.
4.Place the strip of plastic into the cake tin making sure it doesn't flap up away from the sides.
5.Melt the chocolate to a thin consistency and spread it around the inside of the plastic and allow to set 100% before applying more layers. Be sure to spread the chocolate all the way to the base. The butter/shortening will keep it from sticking and it will give the cake something to weight the collar down with once you put the cake in.
I find that if I allow the chocolate to set properly before applying each layer there is less "streaking" on the chocolate.
Once all of the layers have been spread and set, gently turn lift the plastic and chocolate tube out of the cake tin. Carefully remove the plastic and voila. You have a cake collar.
6. Place it over the layered and iced cake and pipe in more icing between the gap. ( If your cake is already covered in buttercream, then a bitter sweet ganache could be piped in around it? This is a first attempt after horribly failing the wrap around technique. I piped scribble lines, and realised it would break to easily and just smeared chocolate over it. I was pleasantly surprised by the end result. .
After practising however, I used the technique (minus the scribbling part) and this was the final result.
Ps. The roses were organised from an organic farmer banning all pesticides aside from chilli spray and all stems we wrapped really well before applying to the cake. It was the brides request to put them on so we came to a compromise.