Malzahar beats Kassadin pretty handily. You will only struggle a little bit in the early levels 1-4 because your Space-Aids can't quite AFK-clear the wave yet at these levels. The key here is to not die to him and don't run out of mana by making the most of your Voidlings to help you CS. Landing space aids on Kass while you have a Voidling out also does quite a bit of damage to him because they do physical damage to bypass his magic defenses.
Once you get to Level 5 with the Rank 3 Space Aids you will start to heavily out-farm him, and at level 6 he will never be able to kill you unless you make a big mistake. Just make sure you call out/follow his roams so that he doesn't get fed off your other lanes and get out of control.
In teamfights, you can either land your silence or ultimate on him to stop him from jumping around while your team blows him up. As an AP champ, building a QSS on Kass is not ideal since it doesn't build into anything better. Thus, if you play properly you can counter pretty much all of his moves.