Preheat the oven to 170°C/gas mark 3/325ºF and line a baking sheet with baking parchment or butter paper.
Cream the butter and sugar in a bowl and when you have a light, soft, whipped mixture, beat in the 40g / one-third cup cocoa powder (sieving if it is lumpy). When that’s mixed in, beat in the flour with the bicarbonate and baking powder. Alternatively, just put everything in the processor and blitz, if you prefer.
This mixture is very soft and sticky so it is easiest to form the biscuits wearing disposable vinyl gloves. Pinch off pieces about the size of a large walnut, roll them into balls, then slightly flatten into fat discs as you place them, well spaced, on your baking sheet; you should get about 12 on at a time.
Bake each batch for 15 minutes; even though the biscuits won’t feel as if they’ve had enough time, they will continue to cook as they cool. They will look slightly cracked on top.
Remove the baking sheet to a cold surface and let it sit for 15 minutes before transferring the biscuits to a wire rack, with a sheet of newspaper under it (to catch drips while topping them).
To make the topping, put the cocoa powder, icing sugar, water and vanilla extract into a small saucepan and whisk over a low heat until everything’s smoothly combined. Take off the heat for 10 minutes.
When the biscuits are cool, drizzle each one with a tablespoonful of chocolate glaze – to glue the sprinkles on in a minute – using the back of the spoon to help spread the mixture.
After you’ve iced the biscuits, scatter with some of the Christmas sprinkles, and continue thus until all the biscuits are topped. If you ice them all before sprinkling, you will find the cocoa “glue” has dried and the sprinkles won’t stick on.
Sit back and enjoy the biscuits with your friends and family, or even on your own.