Melt the chocolate in a bowl set over a pan of barely simmering water (make sure the base doesn’t touch the water), or in a microwave in short bursts. Set aside to cool.
Beat the butter and sugar using electric beaters, then beat in the eggs, flour, cocoa powder, baking powder and chocolate. Pour in the milk to make a soft dough, but don’t overmix it. Cover and chill for 1 hr.
Heat oven to 180C/160C fan/gas 4. Line one or two baking sheets with parchment. Put the icing sugar in a bowl. Scoop heaped tablespoons of dough and roll each ball in your hands before dropping it into the icing sugar and rolling it around. Put on baking sheets and repeat, spacing the balls apart.
Bake for about 12-15 mins until the biscuits feel firm when touched. The biscuits should puff up and cracks should open up, too. Leave to cool on the trays, then transfer to wire racks to cool completely. Will keep for up to a week in an airtight container.