In a large bowl, beat together the butter and sugar until pale, fluffy and well combined.
Beat in the vanilla extract and beaten egg, a little at a time, until well combined.
Sift the flour, baking powder and salt into the bowl and mix well until smooth and combined.
Lay a large sheet of cling film (approx 30cm/12in square) onto a work surface and pour the cookie batter into the middle of it.
Bring the far edge of the cling film over the cookie batter and towards you, then use the side of a palette knife to press the batter against the length of cling film. Wrap the cling film around the dough to form a sausage shape 7-8cm/3-4in wide. Twist the ends to secure and chill in the fridge for one hour, or until firm. (NB: At this stage, the cookie dough can be frozen until needed.)
Preheat the oven to 200C/400F/Gas 6.
When the cookie dough is firm, cut 1cm/½in slices from it and arrange them on a baking tray with enough space around each to allow for the mixture spreading.
The cookies can be cooked as they are. Alternatively, sprinkle over toppings of your choice, such as chocolate chips, nuts or crystallised ginger before cooking.
Transfer the cookies to the oven and bake for 8-10 minutes, or until the edges are pale golden-brown.
Set the cookies aside for 1-2 minutes, then transfer to a wire rack to cool.