Petri Dish Cookies

Directions

1. In a large bowl, cream the butter and sugar until light and fluffy. Beat in egg and vanilla until just combined. Slowly beat in flour, baking powder, and salt until it comes together into a dough.
2. Shape dough into two discs, wrap in plastic wrap, and refrigerate for at least 2 hours. (Well wrapped, the dough can be refrigerated for up to 3 days or frozen for up to 2 months.)
3. Center a rack in the oven and preheat the oven to 350 degrees. Line two baking sheets with parchment or silicone mats.
4. Working with one packet of dough at a time, on a clean floured surface roll out the dough evenly to a thickness of about 1/8-1/4 inch. Cut out the cookies (I used a 2-1/2 inch diameter round cookie cutter). Pull away the excess dough, saving the scraps for rerolling, and carefully lift the rounds onto the baking sheets with a spatula, leaving about 1-1/2 inches between the cookies. After you’ve rolled and cut the second packet of dough, you can form the scraps into a disk, then chill, roll, cut and bake.
5. Bake the cookies one sheet at a time for 8-10 minutes, rotating the sheet at the midpoint. The cookies should feel firm, but they should not color much. Remove the pan from the oven and let them rest for 1 minute before carefully lifting them onto a rack to cool to room temperature.
6. Repeat with the remaining dough, cooling the baking sheets between batches. Allow cookies to cool completely before decorating.
7. While the cookies cool make the icing: mix all ingredients on low speed for 7-10 minutes or until the icing loses its shine. Add more water by the teaspoon if it appears too stiff, or more powdered sugar if too thin. At this stage you want to be able to pipe it easily.
8. Color the icing and then cover the container with a damp paper towel. It is key when working with royal icing not to allow it to dry out.
9. Outline the cookies with the royal icing. Place some of the icing into a disposable pastry bag fitted with a #2 tip and coupler and outline the outside of the cookie. Keep the tip about 1/2-inch above the cookie while moving it allows the icing to lay on the cookie more easily. Make sure that the outline is pretty well set before moving on to flooding the cookies (generally by the time  you are done outlining the first ones are already dry).
10. To the remaining icing, slowly start adding a few drops of water at a time, until the icing reaches an almost liquid consistency. The test here is to pick some icing up with a spoon and let it drizzle back into the bowl – the drizzle should disappear into the bowl within 10 seconds. Once you have achieved this, you are ready.
11. Either fill a squeeze bottle with the thinned icing or transfer it to a disposable pastry bag with a 1/4-inch hole cut off the end. Now squeeze in the icing to almost completely fill the inside the cookie. Then take a toothpick and gently use it to distribute the icing to any empty spots. Sprinkle with white nonpareils**.
12. Allow to dry completely before storing. The cookies will keep at room temperature in a tin for up to 1 week. Wrapped well, they can be frozen for up to 2 months.