Preheat oven to 180°C/350°F. Grease a shallow 24cm square cake pan and line base and sides with baking paper, extending the paper 5cm above sides.
2
Beat butter and sifted icing sugar in a small bowl with an electric mixer until smooth. Stir in 1 cup (150g) of the sifted flour. Press mixture evenly over the base of pan.
3
Bake about 15 minutes or until browned lightly.
4
Meanwhile, whisk eggs, caster sugar, remaining sifted flour, rind and juice in a medium bowl until combined. Pour egg mixture over hot base.
5
Bake slice about 20 minutes or until firm. Cool in pan on a wire rack before cutting into squares. Serve dusted with extra sifted icing sugar, if desired.