Think it might be a typo! Should be plain flour.
Another recipe:
175g plain flour
50g cornflour
50g caster sugar plus extra for sprinkling
115g unsalted butter, chopped.
Preheat oven to gas 3
Sift the flour, cornflourinto mixing bowl.
Rub butter into the flour until it binds and you can knead it into a soft dough.
Cut into shapes or press into a greased and floured tin.
Bake for 35 - 40 mins until pale golden in colour.
When cooked sprinkle the top with caster sugar.