Preheat the oven to 220°C (200°C fan) /425°F/gas mark 7. Line two baking sheets with baking parchment. Place the butter and sugar in a mixing bowl and beat until very light and fluffy.
Gradually beat in the egg a little at a time, beating well after each addition. Add the vanilla extract if using. I like to add it when serving the biscuits plain.
Sift the flour into the bowl and gently fold in.
Spoon the mixture into a piping bag fitted with a 1cm (½in) nozzle and pipe 8cm (3in) lengths onto the baking sheet, allowing space for the mixture to spread.
Sprinkle with some pistachio nuts or cocoa nibs, if using.
Bake in the centre of the oven for 6–8 minutes until golden brown around the edges. You may need to do this in batches.
Allow to cool on the baking sheet for a few minutes. Loosen the biscuits carefully with a palette knife and transfer to a wire rack to cool completely and become crisp.
Notes
Store in an airtight container for up to 5 days.Nutrition information is approximate and is meant as a guideline only. (does not include optional ingredients such as the pistachios and cocoa nibs)