Preheat the oven to 180C/350F/Gas 4 and line two baking trays with baking parchment.
Put the butter and icing sugar into a bowl and beat using an electric hand whisk until pale and smooth. Beat in the flour, one half at a time. Add the salt and vanilla extract and then beat in the egg whites.
Spoon the mixture into a piping bag fitted with a 1cm/½in round nozzle and pipe straight lines of the mixture, about 8cm/3in long, onto the paper. Leave 5cm/2in between each biscuit as they will spread during cooking.
Transfer the trays to the fridge for 15-20 minutes, or until the dough is firm to the touch.
Bake the biscuits for 10-12 minutes, or until light golden-brown around the edges. Use a palette knife to transfer to a wire rack to cool (so they don’t stick to the tray).