I max E, put a point in to W, then max Q second. E is amazing harass as its cast range is comparable to every mid laner's AA range.
I typically start with a sapphire and can grab a catalyst on first back, meaning i can out sustain in lane most of the time just off of its level-up proc and my E doing nearly an extra ignites worth of damage each time I add a point to it.
From there its an easy sorc boots on next trip to the fountain, which allows you to not only clear waves super fast with a Q - E on the waves, but really put the hurt on the other laner if they try to punish you for it - you'd be surprised at how often they will stay in range of a minion dying of the space aids and catch it themselves -
Once you have sorc boots and ulti its pretty easy to roam and force out other laners, even if you dont pick up a kill/assist.
Naturally I finish the Rod of Ages ASAP after sorc boots, but it can be more useful to pick up that +30ap/+10% cdr tome before you drop the 1600g on finishing rod if you happen to fall behind in lane, or the need to help your top/bot lanes outweighs the need to kill the mid laner.
Once you have the Rod + Sorc boots, youre pretty much set. From there I tend to build to the situation. If the enemy team is tanky or you notice a carry is building towards MR, Voidstaff is a great next item. I typically prefer Liandry's for its synergy with Rod, my E poke, and Sorc boots penetration. Now that you'll have health, pen, and plenty of damage over time, Rabadons is the obvious next purchase. Because malzahar is a mid-game oriented champion, the large boost of AP you get will help propel you in to late game as the other champions health/resists begin to out scale your typical damage output.
When you want to rekt someone foolish enough to try and take you out, drop a nice E - Q combo on them. IF the silence lands, I will drop the W underneath them and use the ultimate, followed by another E as the channel wears off for an ez game ez life. IF the silence doesnt land, just keep running and dropping your E on them until they realize they wont win an engage anymore at that point.
^^ and for all of this it is EXTREMELY useful to be smartcasting everything.
Oh and if you can while in lane phase, throw an auto attack, smartcast E, and follow with another auto attack every time you poke. Malzahar's auto attacks are nothing to laugh at, and if you want you can even try the AD malz build that works incredibly well. I always use it against Diana in lane, because she begins to outscale Malz incredibly well after level 9 if you go straight AP.