Preheat oven to 180°C or 160°C fan. Line 2 large baking trays with baking paper. Place flour, icing mixture and cocoa powder in a food processor. Add the butter and using the pulse button, process in short bursts until the mixture resembles breadcrumbs. Add almost all the milk and process again in short bursts until the dough is evenly moistened and comes together in small clumps (pinch it in your fingers and add a little more milk if it seems dry). Turn out onto a clean work surface and gather the dough into a ball.