Make the cookies: In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt. Set aside.

In a large bowl, using a handheld mixer or stand mixer fitted with the paddle attachment, beat the butter and sugar together on high speed until smooth and creamy, about 2 minutes. Add the egg and vanilla extract and beat on high speed until combined, about 1 minute. Scrape down the sides and bottom of the bowl as needed and beat again to ensure everything is fully combined.

Add the dry ingredients to the wet ingredients and mix on low speed until just combined. The dough will be soft. If the dough feels too soft or sticky for rolling, add 1 extra tablespoon of flour.

Roll the dough: Divide the dough into two equal portions. Dust two large pieces of parchment paper or silicone baking mats with cocoa powder or flour. Place one dough half on each sheet. Using a rolling pin lightly dusted with cocoa powder or flour, roll out the dough to about 1/4-inch thickness. Use more cocoa powder or flour if the dough becomes too sticky. The rolled dough can be any shape as long as it's evenly 1/4-inch thick.

Chill the dough: Lightly dust one of the rolled-out doughs with cocoa powder or flour. Place a sheet of parchment paper on top to prevent sticking. Stack the second rolled-out dough on top. Cover the dough with plastic wrap or aluminum foil and refrigerate for at least 1-2 hours, or up to 2 days.

Preheat oven & shape cookies: Once chilled, preheat the oven to 350°F (177°C). Line 2-3 large baking sheets with parchment paper or silicone baking mats. Carefully remove the top dough sheet from the refrigerator. Using cookie cutters, cut the dough into shapes. Re-roll the remaining dough, lightly dusting your work surface and rolling pin with cocoa powder or flour, and continue cutting out shapes until all the dough is used. If the dough becomes too warm or soft, stop and return any unused dough to the refrigerator for 10 minutes to firm up. Repeat the cutting process with the second half of the dough. (Note: The dough scraps can be re-rolled, so you’ll get a lot of cookies from them.)

Bake & cool: Arrange the shaped cookies on the baking sheets, spacing them about 3 inches apart. Bake for 11-12 minutes, or until the edges are set. If your oven has hot spots, rotate the baking sheets halfway through baking. Let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely before decorating.

Make either icing: Prepare royal icing or easy glaze icing and decorate the cooled cookies as desired. You can add sprinkles on top of the icing if you'd like. Feel free to color the icing with gel food coloring. Refer to the post above for recommended decorating tools. There's no need to cover the decorated cookies while waiting for the icing to set. For convenience, you can decorate the cookies directly on the baking sheet and place the entire sheet in the refrigerator to help speed up the icing setting.

Enjoy: Serve the cookies right away, or wait until the icing has set. Once the icing has hardened, these cookies are perfect for gifting or sending. Plain or decorated cookies will stay soft for about 5 days when stored tightly covered at room temperature. For longer storage, cover and refrigerate the cookies for up to 10 days.