Preheat the oven to 350?F / 180?C. Line three baking trays with parchment paper.

In a medium bowl, sift together the flour, cocoa powder, baking soda, and salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar until light and fluffy, 3-4 minutes.

Add the egg and vanilla, and beat until combined.

Add the flour and cocoa mixture, and beat on low until just incorporated.

Using a cookie scoop, scoop 2 tbsp worth of mixture at a time (about 45 g balls). Roll into a ball using your hands, then roll generously in sprinkles.

Place on the baking tray, leaving plenty of room to spread (6 cookies per baking tray).

Bake one tray at a time for 11-12 minutes, until the cookies have puffed up and are beginning to crack on the tops, and the edges are set.

Remove from the oven and allow to cool on the baking tray for 15 minutes then transfer to a wire rack to cool completely.