Preheat the oven to 240C/475F. Line 2 baking sheets with nonstick baking paper. Put the butter and sugar in a bowl and cream together until light and fluffy. Beat in the egg. Sift in the flour, add a few drops of vanilla essence and beat until evenly incorporated to a soft, smooth dough. Fill a piping bag fitted with a 5mm/0.25in star or rosette nozzle with the mixture. Pipe on to the baking paper, making whirls 5-6.5cm/2-2.5in in diameter. Press half a cherry into the centre of each whirl. Bake for 5-6 minutes or until light golden. Transfer the biscuits on the paper to wire racks and leave to cool.