Cocoa Butter is a butter that is a hard, slightly yellowish solid at room temperature, as opposed to other butters that are soft or semi-soft at room temperature. It's an edible vegetable fat that is derived from cocoa beans, which in turn gives it a mild chocolate-like odor. There is the option to use a deodorized variation, and this is what I use so as not to interfere with any scent I might add to a product. The choice to use regular or deodorized cocoa butter is a personal one, and there is no difference in any of the qualities.
Cocoa butter is a very stable fat and contains natural antioxidants, which are thought to help combat signs of aging and repair damage to the skin. It has an almost velvety quality to it and melts upon contact with the skin, aiding in keeping the skin highly moisturized.
Cocoa butter helps to add hardness to soap, due to it high percentage of stearic and palmatic acids. Lather from soaps made with cocoa butter tends to be creamy and luxurious as opposed to big and fluffy.