Snopes says it’s true:
Cocoa Mulch, which is sold by Home Depot, Foreman’s Garden Supply and other Garden supply stores, contains a lethal ingredient called ‘Theobromine’.
It is lethal to dogs and cats. It smells like chocolate and it really attracts dogs. They will ingest this stuff and die. Several deaths already occurred in the last 2-3 weeks. Just a word of caution: check what you are using in your gardens and be aware of what your gardeners are using in your gardens.
Theobromine is the ingredient that is used to make all chocolate especially dark or baker’s chocolate which is toxic to dogs.
Cocoa bean shells contain potentially toxic quantities of theobromine, a xanthine compound similar in effects to caffeine and theophylline. A dog that ingested a lethal quantity of garden mulch made from cacao bean shells developed severe convulsions and died 17 hours later. Analysis of the stomach contents and the ingested cacao bean shells revealed the presence of lethal amounts of theobromine.