This chocolate/vanilla shortbread uses the Basic Shortbread recipe as its base.
Prepare 2 batches of basic shortbread, one plain and one chocolate; for chocolate, sift cocoa powder with the flour and salt.
Form each dough into a disk; wrap in plastic and refrigerate for at least an hour (or overnight). Roll out plain dough to 1/4-inch thickness; roll out chocolate dough to 1/4-inch thickness. Place plain dough on top of chocolate dough. Cut into 1 1/2-by-3-inch triangles; refrigerate 20 minutes.
Bake on a cookie sheet until firm and just starting to color, 20 to 25 minutes.
Cookies will keep, in an airtight container, at room temperature for 2 weeks.