How to Make It
Preheat the oven to 350°; position 2 racks in the upper and middle thirds of the oven. Line 3 baking sheets with parchment paper.
Sift the cake flour, all-purpose flour, baking powder and salt onto a large sheet of wax paper. In a large bowl, using an electric mixer, beat the butter and sugar at medium speed until light and fluffy, about 3 minutes. Add the eggs and yolks one at a time, beating well between additions. Beat in the vanilla and milk. At low speed, beat in the dry ingredients until just combined.
Spoon rounded tablespoons of the batter onto the baking sheets, about 2 inches apart. Bake the cookies for about 12 minutes, until the centers spring back when lightly pressed. Be careful not to let the cookies brown or overbake, or they will be dry. Transfer the cookies, flat-side up, to a wire rack and let cool completely.
In a medium bowl, whisk the confectioners' sugar with the boiling water until smooth. Add the vanilla extract and a few drops of food coloring and whisk until the icing is evenly colored. Using a small offset spatula, spread the orange-colored icing over half of the flat side of each cookie.
In a microwave oven, melt the chocolate in a small bowl. Stir the chocolate into the remaining orange icing. Spread the chocolate icing on the other half of each cookie and let stand until set, about 15 minutes. (If the icing becomes too thick, add hot water, 1 teaspoon at a time, until smooth and shiny.)