An acceptable substitute for cocoa is carob. Native to the Mediterranean, Carob is a legume that is highly nutritious, non-allergenic, and virtually fat-free. Its pods can be ground into a flour or powder and it is commonly used in may processed foods as a cocoa or chocolate substitute. 

Carob tends to have a non-sweet, bitter taste. It definitely needs to have sugar/sweetener added to make it palatable. Most recipes that call for the use of cocoa, such as cakes, icing, and brownies will be suitable for the use of carob. To use as a replacement just remember that there is a 1:1 ratio of carob to cocoa. Follow the recipe instructions and substitute an equal amount of carob instead of cocoa. 

Carob can be found in natural foods stores. Chatfield's Carob Powder is a popular brand and may be used in a variety of recipes including baked goods like brownies, cakes, and frosting, as well as a hot-'chocolate' drink. This powder is ground so fine that it produces a rich, creamy result in any recipe. Although it tastes slightly different from chocolate/cocoa most people won't notice.