Pre-heat oven to 180°C
Line 20x30 inch baking tin
Mix butter, flour and icing sugar until it forms a soft dough. Press into base of tin
Bake for 15-20 minutes until lightly golden
Mix castor sugar, lemon juice and rind, custard powder, baking powder and eggs until well combined
Pour over warm pastry base and bake a further 25 minutes until set
Slice when cold
Sprinkle with icing sugar