The problem with trying to make hot chocolate by melting eating chocolate in water is that it doesn't dissolve well, unless you use milk chocolate, which is too sweet. Chocolate is basically cocoa + cocoa butter, so if we deconstruct it then ...
1 oz butter
1 oz (4 tbs) cocoa powder (Callebaut is good)
1 oz (2 tbs) sugar
12 oz milk (skim is fine ... might just have enough fat already ;-)
pinch of salt
Put the butter and sugar together in the microwave to melt.
Stir in the cocoa powder and salt to form a paste.
Add the milk, heat through again in the microwave, whisk to combine well.
Quit whinging about the butter and try it. Butter is about 80% fat, dark chocolate is about 18% fat, so it's equivalent to about 4.5 oz of dark chocolate - that is not extreme.