Whisk the flour, cocoa powder, baking powder, and salt together in a medium bowl. Set aside.

In a large bowl, beat the butter and sugar together on high speed until smooth and creamy, about 2 minutes. Add the egg and vanilla and beat until combined, about 1 minute. Scrape down the sides and bottom of the bowl and beat again as needed to combine.

Add the dry ingredients to the wet ingredients and mix on low until combined. If the dough seems too soft and sticky for rolling, add 1 more tablespoon of flour.

Divide the dough into 2 equal parts. Dust 2 large pieces of parchment paper or silicone baking mats with cocoa powder or flour. Place a dough half on each and roll out to about 1/4-inch thickness.

Lightly dust one of the rolled-out doughs with cocoa powder or flour. Place a piece of parchment on top and add the second rolled-out dough. Cover with plastic wrap or aluminum foil and refrigerate for at least 1-2 hours and up to 2 days.

Preheat oven to 350°F (177°C). Line 2-3 large baking sheets with parchment paper or silicone baking mats. Remove the top dough piece from the refrigerator. Use cookie cutters to cut the dough into shapes. Re-roll the remaining dough as needed.

Arrange shaped cookies on baking sheets 3 inches apart. Bake for 11-12 minutes or until edges are set. Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.

Prepare the royal icing or cookie icing and decorate the cooled cookies. Add sprinkles on top of the icing if desired. Once decorated, allow the icing to set before serving.

These cookies remain soft for about 5 days when covered at room temperature. For longer storage, cover and refrigerate for up to 10 days.