Preheat oven to 350 degrees.

Butter a 9- to 9 1/2 inch springform pan.

Beat butter with an electric mixer until smooth (about 1 minute).

Add brown sugar and beat until light in color and texture (about 2 minutes).

Stir in flour, cornstarch, and salt using a wooden spoon.

Press the dough evenly into the pan.

Press around the perimeter of the dough with a fork.

Prick the dough into 10 equal wedges, reaching to the bottom of the pan.

Bake until lightly browned (about 30 minutes).

Cool completely in the pan.

Remove the sides of the pan.

Cut the shortbread into wedges, following the perforations.