How can I make a chocolate cup that looks like the seaweed on a sushi roll?

"Plastic chocolate" is a form of chocolate used for modeling and shaping decorative elements such as chocolate roses, ribbons and other elements for cakes and desserts.

Take about 1 lb. of bittersweet chocolate and melt over a double boiler. When chocolate is melted, stir in 2/3 cup of light corn syrup. Mix until evenly blended and then set aside to cool. When it firms up, wrap tightly in a heavy gauge plastic bag or wrap tightly in plastic wrap. Let sit for several hours or overnight. No need to refrigerate it, ambient temperature is fine.

Cut the plastic chocolate into smaller size chunks that will be easy to knead. Begin kneading to soften it. If it gets sticky, use a little cocoa powder but be careful not to dry it out. Knead until pliable and then roll out to desired thickness and cut as desired to shape your sushi rolls, decorative elements, etc. You can roll it through the roller of a pasta machine to help get it to an even thickness.

If you're wanting the rough look of nori then press some crumpled foil into it after rolling as previously suggested.

So I made this. It worked out really well. The recipe leaves some room for improvement, but overall, I was happy with the way it turned out.

I rolled out the chocolate, pressed crumpled tin foil in, and then made the nori.

The dish put together.

Dessert for four.

I used reverse spherification of peach puree for the caviar, sliced peaches as ginger, raspberry sauce as soy, rice pudding as sushi rice, and pistachio butter as wasabi. The presentation was great. The flavor was good.

Some things that could greatly help this dish:

1. The pistachio butter didn't work at all. The flavor clashed terribly with the dish. Next time, I think I'll try a mint whipped cream with some green food dye.
2. To get the texture in the chocolate, I crumpled the tinfoil and then uncrumpled it to make the impression. I think this would have worked better if I'd crumpled it in to a cylinder and then rolled it across the chocolate.
3. I left the spheres in a water bath for a while and flavor leached in to the bath (the spheres tasted different upon making and serving). This should be the last step next time.
4. I didn't strain the raspberry sauce. It tasted good, but was too thick as "soy". Straining would have fixed that, I think.
5. The peach slices could have been cut much thiner so that they could get a ginger folded look on the plate.
6. I could have done a more Japanese presentation. Some soy sauce containers with the 'soy' and the 'wasabi' would have been a much better presentation. Unfortunately, I didn't have a set of 4 plates.
7. The chocolate was firm to begin with, but really got soft with handling and being in a hot kitchen. Next time I would make the cups, fill with rice, and then refrigerate until service.

I welcome thoughts or suggestions in the comments.

I don't do much with chocolate, so I don't know if you can mold in it, but I think a mold made of crumpled and flattened tin foil would give a pretty convincing texture.

How about using a fondant instead of chocolate? Roll it thin enough then wrap with a sushi mat to get the bamboo texture onto it.