Preheat the oven to 170 degree, 10 minutes before baking. Lightly oil and line a 20.5 cm/8 inch square cake tin with baking paper. Rub together the flour (175 gms) and butter (125 gms) until the mixture resembles breadcrumbs. Stir in the granulated sugar and mix. Turn the mixture into the prepared tin and press down firmly. Bake in the preheated oven for 20 minutes, until pale golden.
Meanwhile, in a food processor, mix together the caster sugar, flour, baking powder, salt, eggs, lemon juice and rind until smooth. Pour over the prepared base. Transfer to the preheated oven and bake for a further 20 - 25 minutes, until nearly set but still a bit wobbly in the centre. Remove from the oven and cool in the tin on a wire rack. Dust with icing sugar and cut into squares. Serve cold or store in an airtight tin.