Won't work. Sorry. Not unless you serve it hot. The fats in the chocolate are designed to melt around blod heat, hence they melt in the mouth. Until then they are hard and greasy. Not what you want. In addition the cream egg contains fondant, which is very sugary. Not a winner.
The ice cream bit would work, the gums used to stabilise the foam will stabilise a mil shake foam too. You could get the chocolate flavour from cocoa powder. Truth be told though you need a high speed mixer and ideally a homogeniser, say an APV Manton Gaulin, and you're looking at a 3 phase electricity supply which can be a bit complicated.
Better idea: my local supermarket (Morrison) will sell a litre of delicious, fresh, creamy, gloopy chocolate milk for £1. 2 pints of milk in the same store is 86p. There's enough chocolatey loveliness in the chocolate milk to mix it with normal milk if you wish. Eat the Mars Bar separately, and leave the food technology to the food technologists.