Remove some of the milk and dissolve the starch and cocoa powder in it. Put the rest of the milk in a saucepan. Break the chocolate into small pieces and add with the sugar. Warm the milk slowly while stirring until the chocolate has melted. Bring the chocolate milk to the boil, remove from the stove and add the starch milk while stirring. Return to the stove over a low heat and simmer for 2 - 3 minutes until the pudding thickens.
Let the pudding cool, stirring occasionally, until it is less than lukewarm. Add the rum, whip the cream until stiff and fold in carefully.