Look to the cookie.
If you've watched the Seinfeld episode where they discuss the black and white cookie, you are in luck. This post just happens to be made public on Festivus, the holiday that George's dad made up. So it's clearly fate.
Black and white cookies are amazing. They're a cookie typically found in New York City, but I've seen Whole Foods have them from time-to-time. If I see a freshly made black and white cookie in a bakery, typically that is what I'll go for.
It's half chocolate, half vanilla. On top of a soft cookie. You can't go wrong!
The trick is to be steady with your hand while icing. I wish I had a better technique, and maybe someone out there can tell me a better way. But for me, I just slather half of the cookie with white icing, let it set, then spread the chocolate icing on the other side.