1. Preheat oven to 180°C. Line 2 large baking trays with non-stick baking paper.
2. Use electric beaters to beat the butter, caster sugar, brown sugar and vanilla essence in a large bowl until very pale and creamy and most of the sugar has dissolved.
3. Add the egg and beat until combined. Add the chocolate and use a wooden spoon to mix until combined. Sift together the flour and cocoa powder. Add to the butter mixture and mix until well combined.
4. Roll heaped tablespoonsful of mixture into balls and place on lined trays about 3cm apart (you will fit about 8 cookies per tray).
5. Bake in preheated oven for 15-17 minutes or until cooked through, swapping the trays around in the oven halfway through cooking. Remove from oven and transfer to wire racks. Dust with icing sugar and set aside to cool. Repeat with remaining mixture