Place butter and sugar in the bowl of an electric mixer. Beat until well combined and add in the egg and vanilla, mixing well.
Fold in the flour, cocoa, baking powder and cinnamon, mix until a soft dough forms. Cover and refrigerate for at least 2 hours or overnight.
Pre-heat oven to 190°C (170°C fan-forced). Line oven trays with baking paper. Roll tablespoons of dough into balls and toss in the icing sugar so they are well coated. Place onto oven trays allowing space between for spreading. Bake for 12 minutes, remove from the oven and allow to cool slightly before removing to a wire rack to cool completely.