Prepare basic shortbread. Form dough into a disk; wrap in plastic, and refrigerate for at least 1 hour (or overnight). Roll out to a 1/2-inch thickness. Cut into 1-by-3-inch rectangles; refrigerate for 20 minutes. Bake on a cookie sheet until firm and just starting to color, 20 to 25 minutes. Let cool completely.
Make glaze: Coarsely chop semisweet chocolate; melt in a heatproof bowl set over (not in) a pan of hot but not boiling water, stirring with a wooden spoon. Stir in safflower oil (for gloss). Remove from heat, and let cool 10 minutes. Dip 1/3 of each cookie into chocolate glaze, and transfer to a parchment-lined baking sheet. Refrigerate until glaze is set, about 10 minutes. Cookies can be stored, in single layers between waxed or parchment paper, in airtight containers at room temperature for 1 week.