Position oven rack in the lower third of the oven and preheat to 300°F.

Prepare two 7 1/2 inch fluted tart pans or two 9-inch glass pie pans.

Split the vanilla bean lengthwise and scrape the seeds into a small bowl; discard the bean.

In a food processor, combine flour, cornstarch, and salt; pulse until blended.

Add sugar, vanilla seeds, and orange zest; process until blended.

Scatter butter cubes over the flour mixture; pulse 6-7 times, then process for 6 seconds, until the crumbs are fine and powdery.

Process another 5-7 seconds, until the dough holds together when pressed.

Press the dough evenly into the tart pans (or only into the bottom of pie pans).

Press the back of a fork around the edges of each pan.

Sprinkle the shortbread with coarse sugar.

Bake for 40-45 minutes, until it is just barely colored a creamy beige.

Place the pans on a wire rack and let cool for 15 minutes.

Remove the rims of the pans (if using tart pans) and cut each round into 12 wedges while still warm.

Transfer the triangles to the wire rack and cool completely.

Store in an airtight container at room temperature for up to a week.