I discovered this build playing and testing a lot, y builded every combination posible been viable to varus and this one pprove been the more attack speed and damage per second with burst posible.
Even the regular ad build can't deal as much as this ap build does, and that's it why i say viable because varus doesn't lack of attack speed in varus in this build, is the most strong stat that he has and with ap cant burst every champ in game,nashor's gives a lot attack speed and guinsoo's also give a highly boost on your way to farm and fight, for last runaan's also gives same boost as guinsoo's make you hit more target efficiency and applying W marks on more enemys.
I personally played this build +250 matches and is very stable in every match up and every teamfight, gives a lot of damage, burst and with good optionals items can have a good suvirvality also.

I recomend before using this get used to the champ, know how to pop the w marks with e and aply one more to deal the mayor damage posible in the combo.

The basic combo is: 3xAA + E.

An optimal combo is: 3xAA or R + E + WQ.

The optimal full combo is: 3xAA or R + E + 3xAA + WQ(Q max charge).

Have fun playing this build and getting wins and LP.

Att: Bizcochuelo :3