Adapted from Nigella Christmas by Nigella Lawson
Yields: 24-36 cookies
Preheat oven to 325. Line 2 large cookie sheets with parchment paper.
In the bowl of a stand-mixer fitted with the paddle attachment, cream the butter and sugar. Beat in the cocoa, followed by the flour, salt, baking soda, and baking powder. The mixture will be crumbly at first. Scrape the sides and bottom of the bowl and keep beating until it comes together.
Using a small cookie scoop or a spoon, scoop the dough into equal-sized portions and roll them into a ball. Slightly flatten each ball into a disc-like shape (because they don't spread out on their own much) and place on the baking sheet. Bake for 13-15 minutes, or until set and cracked on top, but still soft. Allow the cookies to cool on the pan for about 5 minutes before moving them to a cooling rack to cool completely. Place the cooling racks over parchment paper or newspaper to catch the glaze.
To make the glaze, put a small saucepan over medium heat. Whisk together the cocoa, powdered sugar, hot water, and vanilla until the mixture is smooth and well combined. Set aside to cool for about 10 minutes while the cookies finish cooling.
Using a teaspoon, spoon the glaze onto the center of each cookie, allowing some of it to drizzle over the sides. Immediately follow with the sprinkles (don't wait too long or the glaze will set and the sprinkles won't stick). Repeat with remaining cookies. Allow glaze to set about 30 minutes before serving/storing.