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The process by which M&M/Mars and other candy companies smoothly chocolate-coat their confections is called enrobing. Enrobing was created in 1900 to protect the interiors of the bars from drying out. The process begins when the uncoated centers pass through a curtain of liquid chocolate on a continuous stainless-steel belt. The top and sides of each bar are coated with a thin layer of chocolate. The process is repeated a second time, and then the fully coated bar is quickly cooled and wrapped.
Enrobing is the least expensive way for manufacturers to coat their chocolates. At M&M/Mars, the enrobing machines run around the clock to meet the high demand for their products. Unfortunately, traditional kitchen appliances don't include among them an enrobing machine, so in our case, dipping will have to suffice.
The caramel Twix was introduced in 1977, and peanut butter Twix came along in 1982.
Source: Top Secret Recipes by Todd Wilbur.