Heat oven to 350° F. Butter an 8-inch round cake pan and line with a circle of parchment. In a large bowl, whisk together the flour, cornstarch, and salt; set aside.
Using an electric mixer, beat the butter, sugar, and vanilla on medium-high until light and fluffy, 2 to 3 minutes.
Reduce mixer speed to low and gradually add the flour mixture, mixing until just combined (do not overmix).
With floured fingers, press the dough into the prepared pan and prick all over with a fork.
Bake until golden and firm, 30 to 40 minutes. Let cool.
In a small microwave-safe bowl, microwave the chocolate in 30-second intervals, stirring between each, until melted and smooth. Drizzle it over the cooled shortbread and refrigerate until set, 15 to 20 minutes.
Turn the shortbread out of the pan and, using a serrated knife, cut into 12 wedges.
