Preheat the oven to 180C.

Lightly grease a baking sheet approx 35 x 29 cm and set aside.

Put the flour, caster sugar, cornflour and vanilla seeds into a food processor and whizz briefly to combine the ingredients.

Add the butter and whizz until the dough is fully combined and starting to clump together.

Turn the dough out onto a non-floured work surface and bring it together with your hands. It will be a little crumbly but continue to press the dough together and it will combine.

Shape the dough into a rectangle and roll it out to 1cm thick keeping the shape of the rectangle.

Trim the edges with a sharp knife to straighten them then portion the dough into 2.5x6 cm pieces and place on the prepared baking tray with 2cm space between them.

Bake for 15 minutes or until just starting to brown on the edges.

Remove from the oven and immediately sprinkle with caster sugar using a small sieve.

Leave to cool completely on the tray.

Keep in an airtight container for up to a week.