Cream the butter, sugar and vanilla together until light and fluffy. Add the flour in 2 additions and mix until just combined. Turn out on to the work surface and bring the mixture together with the heel of your hand.
Wrap and chill for 2 hours.
Roll in to a rectangle 1cm thick, keeping the edges as straight as possible.
Divide into 80mm x 80mm squares.
Cut and freeze on trays.
Sprinkle with sugar prior to baking.
Bake from frozen at 165c for 17 mins.