Whip the cream until semi-stiff. Roughly chop the chocolate. Heat milk in a saucepan over low heat. Add chocolate and dissolve in it while stirring.
Put the milk chocolate mixture into a mixing bowl. Fold in 2-3 portions of cream and divide into 4 small bowls. Chill for about 2 hours.
Roughly chop the Peanut Butter Cups. Sprinkle over the mousse just before serving.