1) Preheat the oven to 180C/Gas mark 4. To make the shortcrust pastry, in the bowl of an electric mixer fitted with the paddle attachment, cream the butter and caster sugar until pale. Combine the plain flour and salt and, with the mixer on low, add to the butter until just mixed.
2) Turn out the dough onto a well-floured board and gather into a ball. Flatten the dough with floured hands and press it into a 23 by 33 by 5cm baking sheet, building up a 0.5cm edge on all sides. Chill.
3) Bake the pastry for 15 to 20 minutes, until very lightly browned. Remove from the oven and leave to cool on a wire rack. Leave the oven on.
4) For the filling, whisk together the eggs, caster sugar, lemon zest, lemon juice and plain flour. Pour the filing over the shortcrust pastry base and bake for 30 to 35 minutes, until set.
5) Remove from the oven and leave to cool to room temperature. Cut into squares or triangles and dust with icing sugar.