Plan your cookies: You will need 3 cookies for each completed cookie stack. For 12 completed cookies, roll out dough and cut 36 shapes with the onesie cookie cutter. On 12 of the onesies, use a sharp paring knife to cut out a cavity for the icing. These 12 shapes will be the middles of the cookies stacks.
Bake cookies according to recipe instructions. Allow to cool. Prepare Royal Icing according to recipe instructions.
Thin Royal Icing with water to outline/toothpaste consistency (see tutorial). Put icing in a disposable pastry bag fitted with a plastic coupler and a #2 tip.
Pipe a line of icing around the entire perimeter of the underneath side of a middle cookie piece (one with a cavity carved out). Flip the middle piece onto a bottom cookie and press down gently to adhere it. Repeat with all the middle piece cookies. Set aside to dry.
To decorate the top cookies, outline a onesie shape with the outline/toothpaste consistency frosting. Repeat this for all of the top cookies.
Prepare flooding consistency icing (see tutorial). Transfer this icing to a disposable piping bag with a #2 tip and add some icing to each section on the top cookies. Be careful not to add too much icing that it overflows your piped borders. You may need to use a toothpick to help guide the icing to fill the space. Once you're cookies are flooded, you can let them all dry for an hour or even overnight. The icing will need at least an overnight dry to harden fully, but if you're pressed for time, you can proceed with the process because it will be set up enough after 1 hour for the remainder of this project.
While the icing on your top cookies is setting up, color your flood consistency icing blue or pink with food coloring gel, and then pipe it into the cavity created by the middle cookie on top of the bottom cookie. Repeat for all cookies.
After you judge that the icing on your top cookies is partially set, proceed with piping toothpaste consistency frosting around the border of your middle cookie and then placing your top cookie on top.
Carefully grab the non-iced edges of the cookie and press the cookie down gently to secure the stack together.
With black food gel, draw a question mark on the top of each onesie, if desired.
Once you have your family and friends gathered, pass out a cookie to each person. Tell everyone to take a big bite, or break the cookie in half.