Preheat oven to 400° degrees F. Line 2 baking sheets with parchment paper or silicone liners. Set aside.
In the bowl of a stand mixer, cream together the softened butter and confectioners' sugar with the paddle attachment until light and fluffy, about 2 minutes. Add in the egg and beat well, followed by the vanilla extract. Scrape down the bottom and sides of the bowl if needed. Add in the cocoa powder, baking powder, salt, and flour and mix on low speed until just combined and a soft dough comes together.
Dust a flat work surface with cocoa powder. I like using cocoa powder instead of flour so it doesn't tint your beautiful brown dough white. Roll out half of the dough using a rolling pin until the dough is about 1/4" thick.
Use cookie cutters (I like using small 1-1½" ones) to cut out your favorite shapes and place them on the prepared baking sheets about 1" apart from one another. Reroll the dough and cut more shapes until all the dough has been used.
For miniature cookies about 1-2" in size, bake for 7-9 minutes, rotating pans halfway through baking time. My mini cookies baked at exactly 7:30. For larger cookies, about 2-3" in size, bake for about 8-11 minutes, rotating pans halfway through baking time to ensure even cooking. Allow cookies to cool on the sheets about 10 minutes before transferring to a wire rack to cool completely.
For the royal glaze: In a medium bowl, whisk together the warm water, corn syrup, vanilla extract and confectioners' sugar until glossy and smooth. Divide among bowls and tint with food coloring, if desired. Dip each cookie, one at a time, face down into the glaze and allow excess glaze to drip off. Place back onto the wire rack and immediately garnish with sprinkles, if using. Allow glaze to set and harden before packing or storing the cookies.