Kit Kat is sold all over the world. Japanese original flavors are very popular with foreign travelers. When I went to Kansai international airport, I saw various flavors of Kit Kat, Uji matcha, wasabi, Hokkaido melon etc in souvenir shops. There are 17 Kit Kat which are sold in limited sightseeing places in Japan. I think Kit Kat is a good choice to buy for a Japanese souvenir. Kit literally means "absolutely" and Kat literally means "win or pass an examination" in Japanese. That is why many students buy Kit Kat as a good luck charm.
You can get more information about Kit Kat below the link: