1.Melt chocolate and butter in a small saucepan over low heat. Transfer to a medium bowl.
2.Stir in caster sugar, egg and sifted flour, cocoa and soda until combined. Cover; refrigerate for 15 minutes or until mixture is firm enough to handle.
3.Preheat oven to 180°C (160°C fan-forced). Grease oven trays; line with baking paper.
4.Roll level tablespoons of mixture into balls; roll each ball generously in sifted icing sugar, place on trays 8cm (3¼-inches) apart. Bake for 17 minutes or until a cookie can be pushed gently without breaking; cool on trays.