Beat butter, sugar and golden syrup in an electric mixer until pale, then add egg and beat to combine. Stir in remaining ingredients and refrigerate to rest (2 hours).
Preheat oven to 170C. Form heaped tablespoons of mixture into balls, place on oven trays lined with baking paper and press lightly to flatten. Bake, swapping trays halfway through, until golden (10-15 minutes). Set aside to cool slightly, then serve. Biscuits will keep stored in an airtight container in a cool, dry, dark place for 3-4 days.