Normally what i build playing Cass :)
Last item is situational and of course if you game is bombing swap out the soulstealer for something with more durability
obviously take NB 1st to harass the enemy champs in either solo or Duo lane
I max Miasma to buff the slow after Twin fang has been maxed as to improve the slow so that it's easier to catch up with Noxious Burst and then spam Twin Fang
I have noticed though it can take some time to set yourself up for high damage noxious burst is only to keep your Twin Fang cool down low
In teams fights it's best just to get your ulti off for the damage and anyone facing you is a bonus i found paired with someone like Amumu / Jarven it's critical to the other team also with miasma and Noxious Burst going off too if there rooted
Personally i think Cassiopia is a really under-rated champion and since playing her i have had soooo much fun!
Hope you find this build useful Please be kind with your comments as it is the 1st i have posted :)