In a medium bowl stir together the flour, baking powder, salt and icing sugar. Stir in the butter and vanilla and mix until a stiff dough forms. Pat dough into 23cm round cake pan and prick well with a fork. Sprinkle remaining sugar over dough.
Bake for 30 to 35 minutes in the preheated oven, or until golden. Cut into wedges while warm. Cool on wire rack; separate biscuits.