In a medium bowl, whisk together the flour, cocoa and salt until combined. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat together the butter and sugar until thoroughly combined.
Add the egg, followed by the vanilla extract beating until just combined.
Scrape down the bowl. With the mixer on low speed, gradually add the flour/cocoa mixture and beat until just combined.
Turn the dough out onto a work surface sprinkled lightly with confectioner's sugar. Pat the dough into a disc shape.
Wrap tightly with plastic wrap and place in the refrigerator for at least 30 minutes.
Heat the oven to 350F. Line two cookie sheets with parchment paper.
Place the chilled dough on a work surface that has been sprinkled with confectioner's sugar. Sprinkle the top of the dough and the rolling pin lightly with confectioner's sugar to prevent any sticking.
Roll out the dough to 1/8-3/8" thick, depending on your preference, rotating the dough as you roll to prevent the dough from sticking.
Cut out desired shapes and place the cookies 2" apart on the cookie sheets.
Place the cookie sheets into the refrigerator for 20-30 minutes to chill.
Bake the cookies for 17-20 minutes. Rotate the cookie sheets, halfway through baking. You're looking for the cookies to be soft when touched in the center, but fully cooked. Baking the cookies longer will result in a crispier cookie while baking them to just cooked will keep them soft.
Let the cookies cool on the cookie sheets for 10 minutes, before transferring them to a cooling rack to cool completely.
Make the royal icing:
In the bowl of a stand mixer fitted with the paddle attachment, combine the confectioner's sugar and meringue powder.
Add the warm water, corn syrup and vanilla extract.
Beat on low until everything is blended. Turn the mixer to medium-high speed and mix until the icing forms stiff, glossy peaks, about 5-7 minutes.
Icing can be thinned with water depending on the desired consistency. Gel or powder food color can be used to color the icing. Keep the icing covered with a damp towel to keep from drying out. Decorate your cookies as you wish.
Black cocoa sugar cookie dough will keep in the refrigerator for up to three days. Dough can be frozen for up to a month.