For the uninitiated, a Black & White Cookie is a soft sugar cookie, frosted half with a vanilla frosting or glaze and half with chocolate frosting. They are a staple in New York and are generally oversized. Bakeries ice the cookies with a fondant that dries hard to the touch. Home bakers usually mix confectioner’s sugar and a small amount of boiling water, after which, half the glaze is mixed with a little chocolate and perhaps a teaspoon of corn syrup, and stirred until mixed.
The cookie book Got Milk?, by Nancy Cullen, says a good approximation of a bakery glaze is to mix 2-3/4 cups of confectioner’s sugar with 5 tablespoons of boiling water and 1/2 teaspoon of vanilla extract. The consistency should be a bit runny, but thick enough to spread, she says (thin it with more water, or add more sugar to thicken, if necessary). Once all the cookies are half frosted, stir 2 ounces of melted bittersweet chocolate into the remaining frosting (which is likely to need a little more thinning), and frost the other halves of the cookies.