Step 1: It's all in the cabbage and the chocolate
Good chocolate goes without saying. I chose a Scharffen Berger with 62%. Choose hersheys at your own risk.
Step 2: Sponge cake--the core of the cabbage
Step 3: Temper you chocolate
Step 4: Spread the love--and the chocolate
Take into account what side you want to show. Most of the chocolate leaves you will see on the outside, so put the chocolate on the inside of the cabbage leaf. You want a few chocolate leaves that show on the inside, so cover the inside of these cabbage leaves.
Also in general, you want rounded chocolate leaves. Drape the coated cabbage leaves over and upside-down bowl or push them gently into the bowl depending on which way you want the leaf to bend.
When the leaf is coated, put it on a cookie sheet with wax paper and put it in the fridge. Wait about 8 minutes and then when you remember what you forgot, pull them out.
This is the fun part. Start at the tip of the leaf and pull the cabbage away from the chocolate. Take it nice and easy. You might think that the thick part at the bottom will break, but usually it won't.
Step 5: Just this one last step
Do a dry run with your chocolate leaves. Break parts off if they're too tall for the sponge core like mine were. I tried a hair dryer to shape some leaves to fit better with mixed results. You're never quite sure how the leaf will droop. Also the dryer caused the chocolate to miscolor. You can see a little bit in the picture.
The leaves are held on mostly by gravity, but also by a glaze. Something else to look up somewhere else. Glaze is like an icing that will firm up as it cools and hold the leaves to the cake. If you feel really good, glaze the whole cake and add the leaves. I glazed a bit at a time and added one, sometimes two, leaves. Add the bottom and top leaves before the middle. The top is made of broken pieces of leaves to form the tip of the cabbage. These are overlapped by the larger middle leaves.
That's 'all' there is to it. As you can see, there's a lot you can do wrong and it still looks impressive. This is really my best cabbage cake yet, but my wife and my kids were still impressed in the past that I was crazy enough to try. Enjoy your efforts.