Preheat the oven to 180 C / Gas 4. Lightly grease a loose-bottomed 25cm square tin.
Place the plain flour, sugar, butter and ground ginger in a food processor; process until it all comes together in a ball.
Transfer the mixture into the prepared tin; flatten it out with the palm of your hand or the back of a spoon until it is evenly spread. Prick the shortbread all over with a fork. Cut the shortbread into squares or bars now to minimise crumbling after baking.
Bake for 35 minutes, or until golden. Remove from the oven and cool for 10 minutes, and then carefully retrace the cuts with a knife. Cool completely.