Bring the milk, cocoa powder and sugar to the boil, stirring all the while. Remove from the heat. Break up the chocolate. Dissolve it in the hot milk. Bring back to the boil, still stirring all the time. Pour the rum into the liquid. Add pepper to taste. Pour into heat-resistant glasses, dust with grated chocolate and serve immediately.
HINT. If you wish, you can serve with semi-whipped cream. Place a spoon in the glass so that it does not shatter when you pour in the hot liquid. It dissipates the heat.