[Note: this entry originally appeared, slightly edited, over at my Sun blog. But thanks to a liberal new author cross-license policy, anything that's not Sun-IP related has been released into the wild. So two years after first thinking in public about the joys of black and white cookies, I'm revisiting the topic.]
Long before Jerry Seinfeld made the black & white cookie central to a plot, I’ve been a mass consumer of these uniquely New York desserts. The perfect black and white cookie is about the diameter of your open hand, has a sponge cake like base that is soft (never crunchy, unless the cookie has been in the damaged goods bin for a month), and has a layer of vanilla fondant icing that covers the entire cookie, with the chocolate half iced on top of the vanilla icing base. There’s probably some haute cuisine reason for this, other than the chocolate icing having more opacity than the vanilla, but the experience of two icings at the same time defines the B&W cookie thrill. 
European friends are quick to point out that while black & whites aren’t exclusively a right-coast treat; they appear in German bakeries as “Amerikaners.” I think they should concede the point; these are good old US of A food culture. Coloquially they’re known as “half moon” cookies, in parts of upstate New York (upstate here refers to anything north of, say, White Plains). I tend not to think of them as “Jewish cuisine”, although they’re much more likely to be found in Jewish style bakeries.
Searching for wireless access and a caffeine boost before a customer event not too long ago, I popped into a Starbucks in TriBeCa, treating myself to iced coffee and the literal check-out lane candy: Starbucks branded black & white cookies. They’re undersized (if they were summer flounder, the state of NJ would require them to be thrown back until they reached maturity), but sold in packages of two I can get the daily engineering recommended intake of sugar in one serving. But here’s the horrifying part: the chocolate and vanilla icing do not overlap. They are carefully, precisely, almost mechanically lined up, a perfect demarcation line down the center of the cookie. While it’s nice to see black & whites gaining traction on the west coast (of the Hudson river and parts further west, like Ohio) thanks to Starbucks, I feel like the entire experience is somehow lessened. If I want machine-layered icing, I’ll eat a Ring Ding (or three). Black and white cookies are home-made, they reside in big glass display cases, peering out like neatly stacked waxing (or waning) moons. Some things are not meant to be mass produced, even if they are mass consumed.
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the yin and the yang of baked delicacies. it’s a uniter, not a divider. it is, indeed, diet-breaking worthy. All hail the black and white!
There is a similar Mexican cookie whose name escapes me…that consists of a shortbread style cookie half dipped in milk or dark chocolate. Also yummy.