Cognizant and its root form cognizance used to have no -g- (compare to the related modern French connaissance). The -g- slipped into English some centuries ago and is now standard in both spelling and pronunciation. Both words are also spelled with a -z-, never an -s-.
I have never liked cognizant. It has the auditory pleasure of an unoiled machine, is too popular in corporate lingo, and can always be replaced with the simpler, cleaner, and older alternatives aware or mindful. Its strict definition is knowledgeable of something especially through personal experience, though I doubt anyone anymore uses it in any way but as a direct synonym for aware, knowledgeable, mindful, or conscious—all of which are softer on the palate and cleaner to the ears.