First thing to say about AP malph is that its a risky thing to play him like this and doesnt work against any champion in mid. against AP yi or fizz u have certain problems cause u have just 2 dmg spells and ur ulti has high cd.
if u want to kill anyone with AP malph u need ur ulti if u havent lich bane yet, so be sure u have ulti before u engage a fight.
u have a lot of burst and high cc but u are melee so u need to be In the fight and u have barely chances to escape if ur focused.
but if well played u can do alot of dmg.

rly important to buy with AP malph is kages lucky pick, cause u cant always farm all minions cause ur melee and ur oposite may be ranged, so u gettin gold permanently.
the other items speak for thereselves i think.
Just be sure not to buy rylais because ur q and ur e have a slow yet and ROA gives u way more health and mana which is important for Malph.
For gettin more tanky i play him with Zhonyas cause it gives u amor and a very usefull active. for magicresist u can buy abyssal but i like him way more wizh lich bane and mercury treads but u can also play him with sorcerer boots.
nice other items are:
abyssal scepter for more mr
Void staff to get penetration cause u have no sorcerer boots
morellios evil tome for cd

pros
nice burst dmg
much cc
rly tanky for an ap carry
good in teamfights

cons
no escape spells
ulti has high cd
without ulti useless in early and mid game