Preheat oven to 350 degrees. Place a rack in the middle of the oven. Lightly spray a 9-inch tart pan with a removable bottom with nonstick cooking spray.
In a medium bowl, sift together flour, cornstarch and salt. Set aside. In the bowl of a stand mixer fitted with a paddle attachment, cream together the butter, sugar and orange zest on medium speed until light and fluffy, about 3-4 minutes. Turn the mixer to low and slowly add the flour mixture, just until the dough starts to form large clumps. Stir in the chopped chocolate chunks with a wooden spoon.
Spread the dough evenly into the prepared pan, smoothing the top with a spatula. Using a fork, prick the entire surface of the dough about 1/4 inch deep, and about every 1/12 inches.
Place the pan on a baking sheet and bake until lightly browned, about 20-25 minutes.
Remove from the oven and slice into 8 wedges while the shortbread is still warm. Cool in the pan, then remove from tart pan. Shortbread can be stored in an airtight container at room temperature for up to 5 days.