Here's the thing: If you have a dog, you probably know that it is not good for the dog to ingest chocolate because of the chemicals theobromine and caffeine. What may not occur to you is that those same chemicals are found in the cocoa mulch that many people use in their gardens. One of the reasons people buy cocoa mulch is its sweet smell and this also attracts the dogs and makes them think it's a sweet snack.

The bottom line is that although, according to Snopes (see link below), there has only been one substantiated case of a dog dying from ingesting cocoa mulch, if you have a dog (or a cat, for that matter) it's probably best not to use it in your garden. 

http://www.snopes.com/critters/crusader/cocoamulch.asp