Mix together butter and sugar until light and fluffy.
Add vanilla and mix.
Add in flour and cornstarch and mix well.
Line quarter sheet pan or 9×13 pan with parchment paper that overhangs the sides to easily lift out the bars after they are baked.
Spread dough even in the pan. Place parchment paper over the top and use small pastry rolling pin to smooth out. Chill for 30 minutes.
Use a sharp paring knife to score into 24 bars. Pierce fork holes into each bar. Sprinkle with white sanding sugar.
Chill cookies in the fridge for 5-10 minutes.
BAKE
Bake shortbread at 350° for 30 minutes. Remove from oven, re-score the bars and return to oven for 10 more minutes or until golden brown. Turn off oven and leave shortbread in the oven with door cracked for 5 more minutes.
Let cool. Cut bars with sharp paring knife and store in a container.