This halfmoon cookie recipe hails from Utica, New York and is emphatically different from a New York City style black & white cookie. It's a light chocolate cookie base, with a half vanilla and half chocolate frosting top.
Preheat oven to 350 F. Line a baking sheet with parchment paper or a silicone baking mat.
In a small bowl, sift together the flour, baking powder and baking soda.
Place the butter in the bowl of a stand mixer fitted with a paddle attachment and cream on low speed for one minute to loosen.
Add the sugar and cocoa powder to the butter and beat on medium speed until light and fluffy
.Add the egg and vanilla, beating to combine.
Add half the flour mixture, followed by the milk, followed by the other half of the flour mixture.
Use a large (3 tbsp) cookie scoop to scoop the cookie dough onto the sheet, leaving plenty of room around each cookie as they will spread to roughly 3-inches in diameter.
Bake for 12 minutes until the cookies are puffy and spring back when touched.
Remove cookies to a cooling rack to fully cool.
Meanwhile, make the vanilla buttercream frosting.
For the vanilla frosting:
Cream butter and confectioners' sugar until smooth in a large bowl.
Add the vanilla extract, then gradually add the milk, pour just enough to thin icing to a spreadable but not drippy consistency.
For the chocolate fudge frosting
Make a double boiler by pouring enough water in a small saucepan so that it is about 2 inches deep. Place a medium bowl on top of the saucepan and heat the water gently over medium low heat until it comes to a simmer.
Add the bittersweet and semisweet chocolates and butter into the bowl.
Add the confectioners' sugar, corn syrup, vanilla and salt, and 6 tbsp boiling water and mix until a smooth, stiff paste forms with a rubber spatula.
If needed, thin the icing with additional boiling water until it is spreadable.
Keep icing warm over simmering water until you are about to use it. You can chill and rewarm later in a double boiler over low heat when you are ready to use it, if needed.
To assemble:
Once the cookies have fully cooled and the ganache has a creamy, spreadable consistency, flip the cookies over and use an offset spatula to spread half of the flat side of each cookie with a thick coating of ganache chocolate fudge frosting.
Then spread the other half of each cookie with the vanilla frosting.