Preheat the oven to 170°C (325°F/Gas 3). Sprinkle half of the icing sugar on a baking tray lined with baking paper and top with the puff pastry. Sprinkle over the remaining icing sugar, cover with baking paper, then place another baking tray on top to weight down the pastry and prevent it from rising. Bake for 20 minutes or until the pastry is crisp and the sugar is caramelised. Transfer the pastry to a wire rack to cool.
Break 200 g (7 oz) of the pastry into small pieces and place in a blender. Add a pinch of salt and, with the motor running, gradually add the oil and blend until a fine paste forms. Pour into a 30 × 20 cm (12 × 8 inch) tray lined with freezer film and spread to 4 mm (⅛ inch) thick. Refrigerate for 20 minutes or until set.