New York-Style Black and White Cookies
In the bowl of an electric mixer, beat the butter until very light and has visibly increased in volume, about 10 minutes. Beat in the powdered sugar, vanilla, cinnamon and salt until well combined. Separate the eggs. Add the eggs yolks, one at a time, beating well after each addition. Beat the egg whites until stiff. Combine the flour, cornstarch and baking powder and alternately fold into the butter mixture with the egg whites, beginning and ending with the flour mixture. The batter should be soft, add a little milk if necessary.
Preheat the oven to 180°C (approximately 350°F). Line a baking sheet with parchment paper. Spoon the batter into the piping bag fitted with a large plain tip and pipe generous tablespoon mounds onto the baking sheet, spacing them 2 inches apart. Bake until lightly browned around the edges, about 10 minutes. Then brush with milk, bake another 5 minutes until firm to the touch. Transfer to a wire rack to cool.
Prepare the chocolate cake glaze according to package directions. Place the powdered sugar in a bowl and add just enough lemon juice to make a smooth, thick spreadable icing. Coat one-half of the flat side of each cookie with the white icing and the other half with the chocolate icing. Let set until dry.