Bittersweet Chocolate is a type of dark chocolate.  Dark chocolate is is made by adding fat and sugar to cocoa.  Dark chocolate must contain more than 35% cocoa to be classified as dark.  Dark Chocolate is generally refereed to as semisweet chocolate.  Bittersweet is considered extra dark chocolate, with more cocoa and less sugar than semisweet chocolate.  Cocoa used in chocolate comes in the form of Chocolate Liquor, which most people believe is actual alcohol but really just finely ground cocoa to the point it becomes a  liquid form.  Solidified chocolate liquor is unsweetened baking chocolate.  Fat comes in the form of cocoa butter.

Recently dark chocolate has been trending because of its health benefits.   Bittersweet Chocolate is a good source of epicatechin and gallic acid which are protective of the cardiovascular system.  It also has a high amount of antioxidant compounds which can lower blood pressure.  Bittersweet chocolate also contains the alkaloid theobromine which is a known aphrodisiac and stimulant.  Due to this information it was very difficult to even find a Bittersweet Chocolate bar in the store.  Everything available was 80%, 85%, 90% Cocoa (Dark Chocolate with even less sugar, and not necessarily considered bittersweet anymore).   I eventually found a Ghirardelli Bar: