Heat the oven to 375°. In a large bowl, whisk together flour, baking powder, and salt; set aside. In another bowl, combine sugar and butter, and beat with an electric mixer on medium-high speed until pale and fluffy, about 3 minutes. Add eggs one at a time, beating after each addition, and beat until smooth. Add milk, vanilla, and the reserved flour mixture; beat on low speed until just combined.
Use an ice cream scoop or a 1⁄4-cup measuring cup to divide the dough into roughly 24 portions. Transfer dough portions to parchment paper-lined baking sheets, spacing the portions 2" apart. Bake until cookies are set and lightly browned at the edges, about 15 minutes. Let cool completely.
Meanwhile, make the icing: Whisk together the confectioners' sugar and 1⁄3 cup boiling water in a medium glass bowl to make a smooth glaze (this will be the white icing). Working with one cookie at a time, hold the cookie horizontally, gently gripping it by the edges, and dip the top in the white glaze (you'll get some icing on your fingers); then return the cookie to the baking sheet to let the icing set. Meanwhile, make the black icing by adding the chocolate to the remaining white glaze. Microwave the mixture for about 45 seconds; stir to combine. Dip each white-glazed cookie vertically halfway into the chocolate glaze and transfer to the baking sheet. Let the chocolate glaze set completely before serving.