Place the butter and sugar in a food processor and blitz until just combined. Add the egg and blitz for 30 seconds. Tip in the flour and process for a few seconds until the dough just comes together. (Be careful not to over-process, or the dough will become tough.) Add a tablespoon of cold water if the dough seems too dry. Knead lightly on a floured surface and shape into a flat disk. Wrap in plastic wrap and refrigerate for 30 minutes before using.