The pilot-in-command's failure to maintain control during the initial roll at onset of the stall due to his improper remedial action related to the stall recovery. A contributing factor was initiation of an intentional stall with residual wing ice contamination, resulting in the stall occurring at a higher than anticipated airspeed. An additional factor was the flight crew's failure to ensure all external surfaces were free of contamination prior the stall as required by the airplane flight manual.