Preheat oven to 160°C. Line 2 oven trays with baking paper. Draw a 20cm-diameter circle in the centre of each tray.
Use an electric mixer to beat butter, icing sugar and vanilla until pale and creamy. Sift flour and cornflour over butter mixture. Use a wooden spoon to stir until a soft dough forms. Turn onto a lightly floured surface and gently knead until smooth.
Divide dough into 2 even portions and shape each portion into a 12cm round disc. Place each disc on the lined trays, and using circles as a guide, gently press shortbread out with fingers to form a 20cm round.
Use a fork to gently press the outside of the shortbread, and prick dough with a pattern to decorate. Carefully score 8 triangles into each shortbread. Place in the fridge for 20 minutes or until firm.
Sprinkle the white sugar evenly over each disc. Bake in preheated oven for 30-35 minutes, swapping trays halfway through cooking, until lightly golden and firm to touch. Remove from oven and set aside on trays to cool completely. Cut shortbread into triangles using a sharp knife.
