From cocoa mass to luscious, smooth chocolate generally takes four steps.
Mixing: sugar and cocoa butter are added, and usually lecithin, an emulsifier derived from vegetable fat. Extra cocoa butter keeps the chocolate solid at room temperature and causes it to melt at mouth temperature. Evaporated or powdered milk is added for milk chocolate. The ingredients are mixed in a round machine with a rotating base until dough-like.
Refining: the chocolate mixture is passed through a series of huge rollers to reduce the particle size and create a very smooth material.
Conching: this machine, named for its resemblance to a shell, agitates and heats the chocolate. As it is aerated, the chocolate develops its flavor and becomes velvety smooth. Inexpensive chocolate may be conched 4 to 12 hours; premium chocolate for 3 to 7 days. At this time, cocoa butter and flavoring are added to improve texture and taste.
Tempering: the warm chocolate is stirred and gradually cooled in large kettles, but remains in a liquid state. This process stabilizes various crystalline compounds in the cocoa butter.