If you're not from New York, haven't seen episode 77 of Seinfeld titled 'The Dinner Party,' or don't happen to have a strange obsession with all things New York (like me), you may not have ever tasted or even heard of a black & white cookie.
If it's made right, it's quite delicious.
But don't take my word for it. Go there and have one!
In the meantime, this link describes it much better than I can (the link says episode 74, but it's 77).
Here are parts of the scene from the Seinfeld episode:
Elaine and Jerry at the Royal Bakery…
"ELAINE: Mmm, I love the smell of bakeries.
JERRY: Oh look Elaine, the black and white cookie. I love the black and white. Two races of flavor living side by side [mumble?] It's a wonderful thing isn't it?
ELAINE: You know I often wonder what you'll be like when you're senile.
JERRY: I'm looking forward to it.
ELAINE: Yeah. I think it will be a very smooth transition for you."
"JERRY: Uh, I don't feel so good.
ELAINE: What's wrong?
JERRY: My stomach, I , I think it was that cookie.
ELAINE: The black and white?
ELAINE: Not getting along?
JERRY: I think I got David Duke and Farrakhan down there.
ELAINE: Well if we can't look to the cookie where can we look?"