Place softened butter, vanilla essence and lemon zest in a large bowl. Sift icing sugar on top and beat with an electric mixer until pale and creamy. Sift the flour and corn flour over the butter mixture and beat with an electric whisk till a soft dough is formed. If the dough is still sticky, add in more flour, 1 tablespoon at a time.
Divide the dough into 3 equal pieces, and shape each into a circular disk. Wrap each disk in clingfilm and refrigerate for at least 45 minutes. The dough can be stored in the fridge for upto 2 days, but in that case you will need to soften it slightly at room temperature before rolling the dough.
Preheat oven to 160 C / 320 F (140 C fan forced). Line a baking sheet with parchment paper.
Place 1 portion of dough on a lightly floured parchment paper or silicon baking mat. Use a lightly floured rolling pan to roll out the dough to about ¼ inch thickness (6 mm). In case the dough is still sticky, use more flour or you can place a sheet of parchment paper on top of the dough, and then roll.
Cut cookies using a square cookie cutter. I used the Soffritto cookie stamp cutter for the signature lemon biscuit sandwich look. But you can use any square or round cookie cutter that you have available at home. To prevent sticking to the dough, dust the cookie cutter in flour and then cut the dough. Make sure to use a good cookie cutter and press firmly so that the cut cookies have sharp edges and release easily.
Repeat with the other two dough disks. Any dough scraps can be re-rolled and cut into cookies. If at any time, the dough feels sticky keep it in the fridge or freezer for a couple of minutes to chill.
Transfer the biscuits to the prepared baking sheet, spacing them about 2 inches (5 cm) apart.
Bake until the biscuits are pale golden and cooked through, 11 to 14 minutes swapping the trays halfway through. The cookies will be soft when they come out of the oven but they will firm up after cooling. Transfer to a wire rack and let cool completely.