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.

Something I have done in the past:

I wanted to have a powder as you stated for use at work where I can only access a water cooker. I froze the chocolate so it became very hard. In this state, it is possible to make very fine flakes with a fine grater or similar. You'll need a high quality one or you'll ruin it with the hard chocolate. This is probably as close to powder as it gets. Also wrap a cloth or something around the chocolate as you grate it so you don't warm it up.

In my case, I quickly mixed the "powdered" chocolate with milk powder (also add whatever other dry substance you want in it, such as sugar). Mixing it with all the other dry ingredients prevented it from becoming a large sticky mass pretty well, though I'd avoid it getting warm enough for the chocolate to get soft. Never happened to me, though. It may be no problem at all.

Try using maltodextrin powder.

I'm not entirely sure it would work, but you could melt the chocolate and mix it with maltodextrin powder. Since maltodextrin powder reacts with things like oils and fats, I think it would work well for your chocolate.