Pre-heat the oven to 150C 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, soda and mixed spice and beat them through until just combined then add the 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 15 minutes and then leave to cool on the trays.
Store in an airtight container when cool.
Notes
Note: Don't chill the dough. These are meant to be flat cookies. If you chill it you'll get a big fat part in the middle and very very thin almost-burnt edges. Variations: You can use pecans instead of walnuts. Tip: Look for walnut pieces at your grocery store. They're often a lot less expensive per kilo than whole walnuts and you want them chopped anyway.