Put the flour and 50 g (2 oz) of the butter in a food processor and blend until the mixture resembles breadcrumbs. Add the icing sugar, measurement water and egg yolk and blend to make a soft dough. Wrap in clingfilm and chill for at least 30 minutes.
Roll out the dough thinly on a floured surface to a 21 cm (8½ inch) square. Fit into a 20 cm (8 inch) square loose-based cake tin or shallow baking tin, pressing the pastry down firmly around the edges. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes. Remove from the oven and reduce the oven temperature to 180°C (350°F), Gas Mark 4. Put the whole eggs, caster sugar and remaining butter into a saucepan and heat gently until the sugar has dissolved. Stir in the lemon rind and juice and cook, stirring, for 5–10 minutes until the mixture has thickened slightly. Strain through a sieve into a jug and pour over the pastry.
Bake for 20 minutes until just set. Leave to cool completely in the tin. Transfer to a plate, dust with icing sugar and serve cut into 16 squares.