1. You can use a homemade basic chocolate sponge cake or a store bought one. Do not use anything besides a basic sponge, the drier the better. I made the mistake of using a brownie which made the filling/centre of the buns pretty paste like and gooey (not that it affected the taste in any way)
2. When you roll the dough make sure you do it as tightly as possible because it will swell quite a bit while baking and the open ends may loosen up.
3. If you don't want to make individual collars for each piece you can place the pieces in a lightly greased rectangular baking tin with raised sides. If you place them in a round cake tin you will get buns that are more triangular in shape as they will cram up together when the dough is placed to rise again. If you place them too far apart, they will spread rather rise in height.
4. If you do make collars for each make sure not to make the collars too tight as the dough will seep out from below and overlap on the collar. Foil works best for this.