Pre-heat the oven to 170C (370F) fan-forced and line two baking trays with baking paper.

In the bowl of an electric mixer, beat the butter, vanilla, sugar and egg together until smooth and creamy.

Sift in the flour, cocoa, soda and ginger and beat them through until just combined then add the chocolate and walnuts and stir until incorporated.

Using a level tablespoon of dough at a time, roll the dough into balls and place on the lined trays leaving at least 3 cm between each one.

Bake biscuits for 12 minutes and then leave to cool on the trays for 5 minutes before transferring to a wire rack to cool completely.

Store in an airtight container when cool.