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.