These come out rather like a chocolate shortbread - a little on the sandy side. Be sure to let the glaze cool and thicken slightly or it will drip right off. Adapted from Nigella Christmas by Nigella Lawson
Preheat oven to 350°F Line a cookie sheet with parchment or Silpat.
Cream butter and sugar in a large bowl until light and fluffy. Sift 1/3 c cocoa powder into the mixture and beat well. Sift together flour, baking soda and baking powder, and add to mixture. (Optionally, combine everything in a food processor and mix until combined.).
Form 1 tbsp sized balls. Place on cookie sheet at least 2" apart, pressing down into fat discs. (The dough will be very sticky; gloves may be a good idea.).
Bake 15 minutes. The tops will develop a cracked appearance. Cool 15 minutes, then transfer to a wire rack until completely cooled. (Placing a sheet of newspaper under the rack will help catch drips in the next step.).
For the topping, combine 2 tbsp cocoa powder with confectioner's sugar, boiling water and vanilla in a small saucepan. Heat over low heat and whisk until smooth. Cool 10 minutes.
Drizzle 1 tbsp of glaze on each cookie, spreading lightly with the back of the spoon, then sprinkle with sprinkles or sanding sugar. Work with 6 cookies at a time so the glaze doesn't dry out before applying the sprinkles.