Preheat the oven to 130° /gas 2 and grease a shallow metal baking tray.
Dice butter into small cubes and stir into the flour, sugar and salt in a large mixing bowl.
Using your hands, rub the butter into the flour mixture until it resembles a fine breadcrumb without lumps.
Tip the mix into the greased tin and press firmly into the base with the palm of your hand paying attention to the edges and corners.
Bake for 45 minutes, then sprinkle with icing sugar.
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