Base Preheat oven to 320°F. Line a 23cm square pan by placing a large sheet of baking paper into the tin pressing into the corners. Do not cut the liner as the topping is very runny and will leak out while baking. Place flour and icing sugar into a large bowl and grate the cold butter into the flour then use your fingertips to rub the butter into the flour until it looks like fine breadcrumbs. Add the Heilala Vanilla Extract and lightly stir through. Tip into the prepared tin, spread out the crumbly mixture evenly then press down using your hands or the base of a flat bottomed glass until it is firm and flat. Bake for 15-20 minutes until very lightly brown. Start preparing the filling.
Filling Pour sugar into a large jug, add the lemon zest and lemon juice. Stir together. Add eggs and vanilla and beat together until well combined. Add the flour and whisk until smooth. Pour over the base and bake for another 30-35 minutes until the top is lightly browned and the center does not wobble. Allow to cool and cut into squares or bars. Dust with icing sugar and serve.