In top of a double boiler, combine chocolate, cocoa, rum, pumpkin spice and butter. Melt over simmering water, stirring constantly.
In a small bowl sprinkle unflavored gelatin over the cold water; let stand for 10 minutes. Add the boiling water and stir until gelatin is dissolved. Stir mixture into the cooled chocolate.
Beat whipping cream with an electric mixer until the mixture is stiff. Add the chocolate mixture and beat until well blended.
Spoon the pumpkin chocolate mousse into dessert dishes. Chill for at least 1 hour before serving.