Petri Dish Cookies

Many bakers make gorgeously decorated cookies that look too immaculate for a regular day person to accomplish. It’s this thing called royal icing, and it involves outlining, flooding and a bunch of little decorating tools. Sounds kind of daunting, which is why it took me so long to try for myself.

Ever since I saw a picture of these petri dish cookies, the lab nerd in me knew I had to join forces with my baking-side to make them. Since our lab just moved into a new facility and the ribbon cutting ceremony is scheduled to be today, I figured it was the perfect time to conquer my fear of royal icing. But I wasn’t about to do it without a little help. Thanks to Michelle at Brown Eyed Baker and her wonderfully helpful tutorial on “How to Decorate Cookies with Royal Icing“, I bring to you these slightly unappetizing-looking, yet completely adorable (okay maybe I’m alone on that one) petri dish cookies!

Although there are a lot of steps, it’s pretty simple when you break it all down (and assuming you have all the necessary tools).


Makes approximately 30 3-inch diameter cookies


2 cups all-purpose flour

1/2 tsp salt

1/2 tsp baking powder

1 stick plus 2 Tbsp (10 Tbsp) unsalted butter, at room temperature

1 cup sugar

1 egg

1 egg yolk

1 tsp pure vanilla extract

1 batch royal icing (recipe below)

White nonpareils


  1. In a medium bowl, whisk the flour, salt and baking powder together.
  2. Working with a stand mixer, perferably fitted with a paddle attachment, or with a hand mixer in a large bowl, beat the butter at medium speed for a minute or so, until smooth. Beat in the sugar and continue to beat for about 2 minutes, until the mixture is light and pale. Add the egg and yolk and beat for another minute or two; beat in the vanilla. Reduce the mixer speed to low and steadily add the flour mixture, mixing only until it has been incorporated – because this dough is best when worked least, you might want to stop the mixer before all the flour is thoroughly blended into the dough and finish the job with a rubber spatula. When mixed, the dough will be soft, creamy and malleable.
  3. Turn the dough out onto a counter and divide it in half. To make roll-out cookies, shape each half into a disk and wrap in plastic. Chill for at least 2 hours. (Well wrapped, the dough can be refrigerated for up to 3 days or frozen for up to 2 months.)
  4. Center a rack in the oven and preheat the oven to 350 degrees. Line two baking sheets with parchment or silicone mats.
  5. 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 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. (This is a soft dough and you might have trouble peeling away the excess or lifting the cutouts; if so, cover the dough, chill it for about 15 minutes and try again.) 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.
  6. Bake the cookies one sheet at a time for 7-8 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.
  7. Repeat with the remaining dough, cooling the baking sheets between batches. Allow cookies to cool completely before decorating.
  8. 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).
  9. 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.
  10. 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.
  11. 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.



4 cups powdered sugar

2 Tbsp meringue powder

5-ish Tbsp water

1/8 tsp Buttercup Yellow icing color (this color is extremely close to the YPD media we use to grow yeast in lab)


  1. 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.
  2. 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.

Source: Sugar Cookies from Brown Eyed Baker, originally from Baking: From My Home to Yours by Dorie Greenspan, pages 146-147. Royal Icing and Tutorial from Brown Eyed Baker.

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