Why Ad Malzahar you ask. well its simple. Malzahar's Voidlings scale off of a 1.0 bonus ad. This build is to take advantage of this fact.

The following if directly from the LoL wiki.

After casting 4 spells, Malzahar summons a voidling for 21 seconds. Voidlings have 200 + (50 × level) health and 20 + (5 × level) + (+1.0 per bonus attack damage) attack damage (bonus autoattack damage isn't noticed in tooltip or voidling's stats, but it affects damage done by voidling). The voidlings benefit from Malzahar's armor penetration.
Voidlings grow after 7 seconds (+50% base damage and armor), and Frenzy after 14 seconds (increased attack speed).
Voidlings' armor: 30 (45 after 7 seconds)
Voidlings' magic resistance: 50
Voidlings' attack speed: 0.831 (1.496 after 14 seconds)
Voidlings' movement speed: 451
Voidlings will always attack the target affected by Malzahar's most recently cast Malefic Visions. If there are no targets affected by Malefic Visions, it will attack random targets.

The main things to remember with this build is this.

1. You can spawn multiple voidlings at once, making your Dps that much stronger.

2. Crit and Attack Speed don't matter on AD Malz since voidlings don't scale with it.

3.Voidlings are Effected by Armor pen and Bonus Attack Damage.

4.Keep track of your voidling as they are most of your dps.

5. If you can, get blue buff, as you need the most CDR you can have

playing ad Malz your job is to make it possible for your voidlings to reach there target. this is done in several ways. They include Exhaust, frozen mallet auto attacks and your Ulti. also for champions with Flash or flash like abilities use your Q.

just some notes

math is down using this build

at lvl 18
110 basevoidling atack at 18

239( bonus AD)+110=349 total attack damage per voiding.

10 attacks in the first 14 sec 9 attacks in the last 7 sec

Max lifetime Damage output of 6631 over 21 sec

thats a rough average of 315.762 DPS

3 voidlings damage over a 7 sec team fight.
2 in pre AS buff state 1 after.

The first 2 get off 5s attacks each. and the last gets off 9

that means 3 voidlings in 7 sec roughly match the damage of 1 voidling over 21 sec.

reaching 947.286 DPS