Flavanols from dark chocolate could be beneficial for insulin resistance.

Note, the flavanols would have to actually be there, some processed chocolates may have lost their flavanols, and therefor have lost their ability to affect nitric oxide dependent processes.

http://www.ncbi.nlm.nih.gov/pubmed/16027246

http://www.medsci.org/v04p0053.htm

Found a little more info about flavanols

The U.S. Department of Agriculture’s Database of Flavonoid Content reports that unsweetened baking chocolate has 206 milligrams of flavanols per 100 grams of chocolate, dark chocolate has half that amount, and flavanol content drops down to 15 milligrams per gram in milk chocolate.

just in case anyone was going to try it