How do I make instant chocolate powder from a chocolate bar?
You cannot easily create a powder from real chocolate, which contains a great deal of cocoa butter, making it very difficult to powder.   While you can chop it finely, that still may be less than ideal for quickly creating hot chocolate.

Instead, based on an idea from Cook's Illustrated, you can make a thick ganache to use as essentially a hot chocolate concentrate, that can be kept in the refrigerator for a few days, or the freezer for a couple of months:


Prepare a gancahe with 12 ounces of chocolate to 1 cup of heavy cream, with 1/4 tsp of salt.
Cool it and then scoop it into single serving portions (about 3 tablespoons) to store, wrapping each individually.   
To make the hot chocolate, heat one portion with one cup of milk (microwaving is especially convenient) until it is hot.


More traditionally, for an instant powder mix, you would combine cocoa powder (often dutched), powdered sugar, and possibly other seasonings--if you want it to work with a water base, you would also include dried milk powder.   Many recipes are easily available on the internet.