When the ice cream is slightly more frozen than when it came out of the ice cream maker, but not hard, remove it from the freezer (depending on your freezer, this may take anywhere from an hour to several hours).
Turn over one sheet of your cooled shortbread so that the flat side faces up. Spread roughly half of the ice cream onto the surface of the shortbread, spreading it to about ½ inch from the edges. Place the other rectangle of shortbread, flat side down, on top of the ice cream. Wrap the entire thing tightly in plastic wrap and freeze until hard. The ice cream will squeeze out to the edges, and that’s what you want it to do.
When ready to serve or pack up for later, remove this ginormous ice cream sandwich from the freezer, unwrap, and with a large, sharp chef’s knife, cut into 9 3-inch x 2-inch bricks. Serve immediately. Or, to serve later, wrap in parchment, securing the paper with twine or tape and store in the freezer, or take them wherever you’re going in a well-insulated ice chest.