Not Updated For Current Season
I tend to build Kass with as much survivabilty as I can, this allows me to be as ballsy as I want. Tower diving to get kills with no worry, and even Riftwalking in to a group of champs to assasinate one, and living to Riftwalk out again.
I go straight Magic Pen. for Reds, these allow your early game spells to hit hard, and for your spells to stay potent once mid to late game hits and MR gets higher.
I go flat MP5 for yellows, these, coupled with the meki pendent allow you to constantly spam your skills to last hit minions in the beggining, and still allowing the mana to harrass if oppportunity arises. Although if you like going with the flat mana item first, or are a bit slow building up the Tears passive, MP5 per level will give you more mana regen starting at level 5.
I go flat CDR for blues, coupled with the standard 9/0/21 masteries this gives you a pretty decent CDR starting from level one, allowing you to get more last hits in the laning phase, and allowing you to both riftwalk more often (escapes and pursuits) and Nether Blade more often (extremely useful for building up AA passive, and Force Pulse)
I go flat HP for quints, these give you a nice boost to starting health, allowing you to be a bit more daring with harass before level 6, and get away with tower diving at level 6. If you dont like the idea of health, feel free to go Magic Pen or AP for more potent early game spells.
For masteries I go the standard 9/0/21 for casters, putting one point in improved exhaust, as it is one of my main summoner skills. I also put 4 points in increased mana instead and 3 in increased XP, to further aid early game spamming, and to ultimately increase late game AP with the AA.
I start the game with a Meki Pendant and two health potions. The meki pendant, coupled with MP5 runes allows spamming of skills early game, facilitating farming, accelerating your build. The health potions allow some leniency during early laning harrasing, in case you take some damage, or come off the bad end of a gank. Once I have enough money to buy a Tear of the Goddess I go back at the first safe chance, making sure not to leave my tower undefended. The early Tear is a great boon come mid game as the spamming you do to keep Force Pulse up, and last hitting on creeps should bring your mana up quite fast. Next item are Sorc Shoes, the early magic pen. makes your early spells hit hard, and late game spells stay potent. Now finally you can get your first AP item! Archangels Staff, built from your Tear this will provide a great deal of AP in the beginning because of the passive (3% of your mana is converted to AP), if you have been spamming sufficiently this should be a decent amount. Next I start building a Rylai's, going with a Giants belt first for decent health and survivabilty, and then the AP parts. This item is great, giving all your spells a slow, while not extremely useful since you have Force Pulse as an AoE slow. But there are the times that a slowing Riftwalk come in handy. After this I build an Abyssal Scepter, building the MR item first, for survivabilty, and AP second. I love this item, it provides a decent amount of MR, AP, and lowers all enemy champs MR by 20, essentially giving everyone on your team Sorc Shoes. I build a Zhonya's Hourglass next, again building survivabilty first and AP second, the armor is the last peice in the survivabilty build, now having protection in all realms: Sustained damage taking with health, magic resist, and physical resist in armor. Finnaly I build a Rabadons, drasticly increasing your AP and making your spells hit absurdly hard.
For skills I like to take Null Sphere first, it has a decent range to last hit, and silences for a great harrass. After this point I prioritize Force Pulse, as it gains more damage per skillup than Null Sphere. However I make sure to take one point in Nether Blade for a second way to gain mana if I should ever run out. It also is a great way to bring up Force Pulse and Tears passive. Of cource, take Riftwalk whenver its available.
For summoner spells I take Exhaust and Ignite. This is an extremely deadly combination for getting first blood. The Ignite also helps take down a hero with lifesteal, and the Exhaust helps with reducing an atttacking champs damage, as well as slowing them quite significantly. That coupled with Kass's awesome pursuit skills ensures they wont get away. However, cleanse and calrity are both viable options (although I suggest having at least one offensive summoner spell) Cleanse helps Kass escape anything, getting rid of the stuns, slows, ect. that would prevent him from Riftwalking away. And Clarity is an alternative to early game mana regen if you prefer something a little more offensive.
I tend to stay quite restricted before level 6, staying back to last hit creeps getting as much money as possible before level 6. Once level 6 hits and I get Riftwalk I go all out as soon as a chance arises. Wait for them to push a little way, making sure I have force pulse up, and Riftwalk in, Exhaust, Ignite, Force Pulse, and Null Sphere. If that doesnt kill them then pop Netherblade and smack em a few times. past this point I gank at every opporunity, with the tankiness in this build you can afford to stay so offensive throughout the game, increasing survivabilty as your enemies increase their damage.