In a large bowl, stir the butter until pale and smooth, a bit like a mayonnaise texture. Then, add sugar, one half at a time. Mix well.
In the same bowl, add half part of egg white and mix well, then add the half part of flour and mix well. Add remaining egg white, salt and vanilla extract and mix it up, then add the remaining flour and mix well.
Spoon the mixture into a piping bag fitted with a ½-in/1-cm round nozzle.
Pipe straight lines of the mixture, about 8cm/3in long, onto the paper. Leave 2-in/5-cm between each biscuit as they will spread during cooking.
Bake the biscuits for 10-12 minutes at 330°F/170°C, 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. Enjoy!