Combine powdered sugar, corn starch, vanilla (optional) and water until icing like consistency is formed.
Remove about three tablespoons worth of the white mixture, and mix in the cocoa powder or black food coloring. (Depending on what you use, you may need to add a bit more water.)
Divide the two colors into individual plastic baggies, or piping bags fitted with large and small round tip. Using a jar to line with plastic bags helps to get the icing scooped in more easily.
Twist and secure the bags to close. Cut a hole in each bag (small for the white, tiny for the black).
Pipe the white in circles onto parchment paper; using a baking sheet or something flat underneath for moving it later.
Pipe small black dots in the centers of the white, before they are completely set.
Let air dry overnight until hardened. Don't go over a 1/4 inch diameter, or they may have trouble drying out.
Store at room temp, and decorate as needed.