Combine dry ingredients (flour, cocoa, espresso powder [see note], baking powder, and salt) in a medium bowl. Set aside.
In the bowl of a stand mixer, beat the butter until smooth and creamy. Add sugar and beat until light and fluffy, scraping down the sides of the bowl as necessary.
Add egg and extracts (see note on chocolate extract) and mix to combine. Gradually add the dry ingredients and mix until just incorporated.
Divide dough into two equal parts. Roll each half to about ¼" thick between two sheets of parchment paper. Transfer the rolled-out dough to a cookie sheet and refrigerate for at least 1-2 hours (chilling the dough is critical!).
After dough has chilled, preheat oven to 350° F. Line cookie sheets with silpats (see note) or parchment and set aside.
Cut shapes out of chilled dough. You may re-roll the scraps, just make sure dough doesn't get too warm.
Bake cookies for about 10 minutes, rotating the cookie sheet halfway through baking. Cool the cookies on the baking sheet for at least 5 minutes before removing to a wire rack to cool completely.
For Icing:
In the bowl of a stand mixer, combine water, meringue powder, and cream of tartar. Using the whisk attachment, beat mixture until soft peaks form. Stop the mixer and scrape down the sides as necessary.
With the mixer running on low speed, gradually add the powdered sugar. Beat mixture until combined.
Color royal icing as desired. I outlined my cookies in royal icing. To frost the rest of the cookie, I added enough water (you won't need much!) to make the icing a pourable consistency, and "flooded" the cookies with the remaining icing.
Royal icing dries out quickly, so be sure to keep it covered with plastic or inside squeeze bottles (see note) if you're not using it right away.
Frost/decorate cookies as desired.Enjoy!
Notes
Note on espresso powder: I've found it difficult to find espresso powder, especially one that's fine in texture and isn't already as hard as a rock. I use King Arthur Baking's brand, as it's very fine and made specifically for baked goods.Note on chocolate extract: I always add a little chocolate extract to all my chocolatey baked goods! Chocolate extract lends amazing depth of flavor! If you can't find it in your local grocery store, it is available online. Even better, make yourself some Homemade Chocolate Extract for next time!Note on meringue powder: I can never find meringue powder at my local grocery store. You can find it in craft stores and cake decorating supply shops, and you can also find it online [here].Note on silpats: I like lining my pans with silpats because they can be used over and over again and the cookies brown more slowly. If you line your pans with parchment paper, just keep in mind that they may be done a little quicker than mine were.