Preheat your oven to 180°c (160°c for fan assisted ovens or Gas Mark 4) and grease of couple of baking sheets with a little excess margarine/butter. Set aside.
Cream together the margarine/butter and sugar in a large bowl until it becomes light & fluffy.
Add the egg & lemon zest, and mix until well combined.
Sift the flour, cornflour and bicarbonate of soda and mix to form a dough - you may need to use your hands. Tip the dough onto a lightly floured worktop, and gently knead until smooth.
Roll your dough to around 5mm thick and cut with a fluted edge cookie cutter. Transfer each biscuit to your pre-greased baking trays and prick each one with a fork on top.
Bake in your pre-heated oven for around 12-15 minutes until the edges of the biscuits are golden.
Remove from the oven and sprinkle a little excess caster sugar over each biscuit. Leave to cool on the baking sheets until cool enough to touch, before transferring to a wire rack to cool completely.