1.Combine chocolate and butter in a small saucepan. Stir over a low heat until just melted. Transfer to a medium bowl.
2.Sift together flour, cocoa and bicarbonate of soda. Stir into chocolate mixture with sugar and egg. Cover and refrigerate, 15 minutes, or until mixture is firm enough to roll into balls.
3.Preheat oven to 180°C (160°C fan-forced). Line 2 oven trays.
4.Roll level tablespoonfuls of dough into balls. Roll in the sifted icing sugar. Place on lined trays, about 8cm apart. Bake, 15-20 minutes. Cool on trays.