How about grating your favourite couverture chocolate, and mix this with some milk powder? With milk powder added (or maybe not if you use milk chocolate couverture), your friends can then just spoon a certain amount of your mixture in their cup and add hot water from the kettle.
I’ve tried using varieties of Callebaut and Valrhona mixed with hot milk. And I’m definitely continuing with Valrhona, not Callebaut. The Valrhona dissolves better in hot liquid is my experience.
I’m working towards the goal of making a mix that gives a nice and convenient hot chocolate “powder” that can be stored for some time, and given away/sold in bags. Due to the fact that everybody in Norway have a kettle, but not everybody have a milk frother (or can be bothered warming up milk in the pan), I want to make a mixture that can be dissolved in hot water. That’s why I’m thinking of using milk powder (dried milk). Main problem here is to get a good quality milk powder without all sorts of additives.