malted balls centers
The ingredients of Whoppers are: 


  SUGAR; CORN SYRUP; PARTIALLY HYDROGENATED PALM KERNEL OIL; WHEY (MILK); MALTED MILK (BARLEY MALT; WHEAT FLOUR; MILK; SALT; SODIUM BICARBONATE); COCOA; CONTAINS 2% OR LESS OF: RESINOUS GLAZE; SORBITAN TRISTEARATE; SOY LECITHIN; SALT; NATURAL AND ARTIFICIAL FLAVOR; CALCIUM CARBONATE; TAPIOCA DEXTRIN


I'm guessing that the sugar, corn syrup, palm kernel oil, cocoa, glaze, and lecithin are in the chocolate coating. This leaves you with a few things to play with for the filling. The wheat flour could be adding some bulk and helping to hold the filling together. However!

Tapioca dextrin... the plot thickens! Tapioca dextrin is a filler made from cassava that has almost no flavor. It is used as a bulking agent in things like sauces and dressings, but it is also used as a binding agent. One of it's main functions is to hold medication together in pills, so that could easily also be holding malted milk powder together in the filling.