Preheat your oven to 350 degrees.

Beat softened butter using an electric mixer on medium speed, until creamy.

Gradually add the sugar, beating until the mixture is fluffy.

Beat in the vanilla extract.

Combine the flour and cornstarch, and gradually add to the mixing bowl. Beat at low speed just until combined.

Press into a 15 x 10 x 1 jellyroll pan (no need to grease the pan first).

Bake at 350 degrees for 15 minutes, then reduce the heat to 325 degrees, and bake for an additional 20 minutes.

Within 5 minutes of removing the pan from the oven, use a sharp knife to cut the shortbread into bars.

Cool completely, and store in an airtight container.