Preheat oven to 180°C. Line a 20 x 30cm sponge roll tin.
In a food processor mix the butter, flour and Chelsea Icing Sugar until it forms a soft dough around the blade. Press into the base of the sponge roll tin. Bake for 15 - 20 minutes until lightly golden.
Mix Chelsea Caster Sugar, lemon juice and rind, custard powder, baking powder and eggs in the food processor until well combined. Carefully pour over the warm pastry base and bake a further 25 minutes until set.
Cut into slices when cold and, if desired, sprinkle with Chelsea Icing Sugar.