Put the flour, sugar, cornflour, salt and lemon flavour (which ever you are using) into a mixing bowl. Stir well to thoroughly combine.
Add the butter and rub into the dry ingredients, using the tips of your fingers. Rub until the mixture resembles fine sandy crumbs. Then tip it into the prepared tin. Press the mixture into an even layer, using the back of a spoon.
Bake for around 25 minutes, until just firm.
Remove from the oven, sprinkle with sugar. Cut into squares with a very sharp knife, while the cake is still warm. (after it might get to crumbly and break while cutting.
Leave to cool before removing from the tin.
Notes
Store in an airtight container up to 6 weeks in a cool (not a fridge) dry place.The recipe freezes also very well up to 3 months. Defrost them by placing them on a rack individually and consume within 5 days.