Hey everyone! So this is not as much of a direct guide/build as much as it is some possible builds highlighting the strengths and weaknesses of Evelynn built either AP or AD. There will be some flaws I'm sure, but these are my suggestions and it's up to you which you prefer based on your play style, but more importantly your team comp versus their team comp with the following conditions:
-What is our ratio of AP/AD users, are we super AD or AP heavy?
-Is their team a kite or engage comp? Do they have a major front line or multiple squishy back liners?
There are questions you should ALWAYS ask yourself when drafting in champ select as you should ALWAYS adapt to what your team needs, not necessarily what you WANT to play as it is not always suitable for the comp.
Where Evelynn is strong, however, is her ability to adapt and build to fill several different roles, whether that be an AP/AD carry or as a tank...one thing will always remain the same though, she has the potential to snowball her team, or herself, early game due to the lack of vision established and the flexibility in gank paths, such as directly flanking from behind or the side/in front of them undetected (lane ganks are a very real thing in terms of walking straight down a lane similar to Rengar ult).

In the next section, let's look at some core items for AP/AD...

AP: With the above build you get solid AP damage and great jungle sustain with your passive and runeglaive's passive, so mana should no longer be a problem. The AP items build in such a way that you will do a reasonable amount of damage while also providing health to have sustain in fights. Rylai's is an amazing item to rush as it will allow you to flank and enemy and stick to them with Hate Spikes + Rylai's. Once you get Liandry's in your build, it will boost your Magic Penetration (along with Sorc Boots if you opt into those) and you will get the additional damage proc's due to the slow of Rylai's. The pair are essentially a much for most AP mages this patch, but even more so on Evelynn to maximize her damage output and resilience when fighting. Finish off tanky with items such as FH and SV as they are just solid items for defensive stats.

AD: I think BorK is a must on Evelynn when building AD because it provides great tank shredding in addition to warrior's armor pen and gives you a nice gap closer/slow when chasing an enemy. I kind of troll with Frozen Mallet because it provides you that same utility as Rylai's more or less would, but with a different effect, all the while your Dark Frenzy (W) will allow you to keep pace with pretty much any fleeing champion. I think the combo of Warrior, Frozen Mallet, and BorK is strong because they all synergize really well in terms of surprising damage and utility. Once again, round off tank with something like FH and SV.

The Masteries and Runes tools are not currently working, but here is a link to my LoLKing profile: http://www.lolking.net/summoner/na/42932623#

Under Masteries look at Page 13 and 14 for two sets of possible masteries to run, both are pretty viable in my opinion. I prefer running 21/0/9 because the amount of damage you take when ganking is typically pretty minimal since you are hard to target before you're already dealing damage to the enemy.

For Runes: The 1st and 2nd page I switch between for any of my junglers. Page 2 is a very standard page if you don't have much IP to dump into Runes, but I like the 1st page for the massive amount of utility it provides; it's a cluster **** of runes, but I like them and it provides and very well-rounded page.

I'm sure the comments of this will be filled with negative feedback, but this is what I run and it works well. I think Evelynn and Elise are super strong picks right now for various/obvious reasons and this is what I pretty much run for either champ (though strictly AP for Elise).