Carob Powder is a good substitute for cocoa powder. Any recipe calling for cocoa powder can use carob powder. Without the caffeine that chocolate has, carob is similar in flavor.
Cocoa powder is sometimes added to baked goods for the purpose of imparting a full, rich chocolate flavor and darker color. It can be used alone or in recipes with your other chocolates and will result in a cake or brownies with a ramped-up, intense chocolate flavor.
Natural cocoa powder is intensely bittersweet while dutch cocoa typically has added alkaline making it less bitter. For this reason, dutch cocoa is preferred when your recipe calls for the use of cocoa powder as a coating. The natural powder is best used in fudge brownies or dark chocolate cake.