A clever idea: hollow bars.

A cooled mould is filled with liquid chocolate and is then upturned again. This allows some of the chocolate to flow out and leave behind an outer, solidified layer along the walls of the mould. This cooled and rigid chocolate shell is turned again and the filling is inserted. This is then shaken to ensure that it is evenly distributed. Next, the chocolate bar’s base is poured over. Finally, any surplus chocolate mass is removed with a scraper and the base-coat is smoothed.