Directions:

COOKIES:
In a large bowl using a hand-held mixer or stand mixer with paddle attachment, beat the butter for 1 minute on medium speed until completely smooth and creamy.

Add the granulated sugar and beat on medium high speed until fluffy and light in color. Beat in the egg and vanilla extract on high speed. Scrape down the sides and bottom of the bowl as needed.

Sift the flour and cocoa powder together in a medium bowl. Whisk in the baking powder and salt until combined. On low speed, slowly mix into the wet ingredients until combined.

Divide the dough into 2 equal parts. Roll each portion out onto a piece of parchment to about 1/4-inch thickness. Stack the pieces (with parchment paper between) onto a baking sheet and refrigerate for at least 1 hour. (Chilling is mandatory.) If chilling for more than a couple hours, cover the top dough piece with a single piece of parchment paper. You can chill up to 2 days.

Once chilled, preheat oven to 350°F (177°C). Line 2-3 large baking sheets with parchment paper or silicone baking mats. The number of batches or cookies will depend on how large/small you cut your cookies.

Remove one of the dough pieces from the refrigerator and using a cookie cutter, cut in shapes. Transfer the cut cookie dough to the prepared baking sheet. Re-roll the remaining dough and continue cutting until all is used.

Bake for 10 minutes -- the cookies will still appear soft. Make sure you rotate the baking sheet halfway through bake time. Allow to cool on baking sheet for 5 minutes, then transfer to a wire rack to cool completely before icing.

ICING:

In a large bowl using a hand mixer or a stand mixer with whisk attachment, beat all of the icing ingredients together on high speed for 5 minutes. (I always start with 6-7 tablespoons of water and then usually need 8 in the end. On particularly dry days, up to 9 or 10 total!)

When lifting the whisk up off the icing, the icing should drizzle down and smooth out within 10-15 seconds. If it's too thick, add a little more water. If it's too thin, add a little more sifted confectioners' sugar.

Icing completely dries in about 2 hours at room temperature. If you're layering royal icing onto cookies for specific designs and need it to set quickly, place cookies in the refrigerator to help speed it up.

When you're not working directly with the royal icing (for example, you are decorating cookies but you still have some icing left in the bowl that you intend to use next), place a damp paper towel directly on the surface of the royal icing. This prevents it from hardening.

Decorate the cooled cookies however you like; add sprinkles on top of icing if desired. Once icing has set, cookies are perfect for gifts, sending, or munching on right away. They stay soft for about 5 days at room temperature.

Make Ahead Tip:
Unfrosted cookies freeze well up to 3 months. Thaw overnight in the refrigerator. Frosted cookies do not freeze well at all. You can chill the cookie dough for up to 2 days (step 3). You can also freeze the cookie dough before rolling out for up to 3 months. Then allow to thaw overnight in the refrigerator. Allow that to come to room temperature for about 1 hour before rolling out and continuing with the recipe as directed