It is a process known as "panning". Panning involves coating candies by rotating them in a coating material in a revolving pan. Panning can be done using syrups and other materials such as chocolate, fats, etc. The principle, briefly, is to coat the centre with a layer of materials, which on evaporation leaves an even layer or shell of dry substance. The chocolate (or chocolate-covered peanut or crispy) centres are colour-coated by rotating them in a revolving pan, while a sugar and corn syrup mixture is added. This process is repeated several times until the chocolate candy have a thin, smooth shell with the desired thickness. Then, the machine specially designed for the purpose gently imprints an "m" on the surface of the fragile candy shell without cracking the thin sugar shell.
