Soften the butter in a large mixing bowl.

Beat the softened butter at medium speed with an electric mixer until creamy.

Gradually add the sugar to the butter, beating well after each addition.

Stir in the vanilla extract until combined.

In a separate bowl, combine the flour and cornstarch.

Gradually add the flour and cornstarch mixture to the butter mixture, beating at the lowest speed until just combined.

Press the dough evenly into a 15 x 10 x 1-inch jellyroll pan.

Bake in a preheated oven at 350°F (175°C) for 15 minutes.

Reduce the oven temperature to 325°F (160°C) and continue baking for an additional 20 minutes, or until the top is lightly browned.

Remove from the oven and let cool in the pan for 5 minutes on a wire rack.

Cut the shortbread into 1-inch squares while still warm.

Store the cooled shortbread in an airtight container at room temperature for up to 1 week, or freeze for up to 3 months.