So Japan has over 200 flavors of KitKat! In the states, I think there's like 4 at any given time... I've probably only tasted about 15 or so of the Japanese flavors. Some are only released at very limited supply, and only at specific times of the year. This flavor is one of my favorites, and is only available during the end of the year holidays.
This flavor is called Butter Cookie. It's covered in white chocolate, with the smell you'd expect of the flavor. But what's so special about these, is that it's one of the few that you can actually put in the oven! But instead of melting, they get crispy! And I can't even begin to describe the flavor enhancement you get after doing this! It's almost orgasmic! I can literally sit down and eat a whole bag of these at once. They are just amazing! You can see how they're baked Here.