Preheat the oven to 350 degrees F. Beat the butter and sugar together, using an electric mixer or by hand, until pale but not too fluffy. With the mixer set on a very low speed, add the flour and salt and beat gently until just combined. Turn out the dough onto a lightly floured work surface and knead into a ball. Using well-floured hands, press the dough into a greased 8 1/2 x 12 inch rectangular baking tin. Chill in the refrigerator for 30 minutes .
