Shortbread is the most delicious cookie there is, period. In no other recipe do 4 basic ingredients--butter, sugar, flour, and salt--create such sensory delight.
Butter an 8 1/2-inch cake or springform pan; set aside. Sift together flour and salt into a small bowl; set aside. Place butter in bowl of electric mixer fitted with paddle attachment. Cream until fluffy, 3 to 5 minutes. Add sugar, and continue to beat until very light in color and fluffy, occasionally scraping down the sides of the bowl with a spatula, about 2 minutes more. Add vanilla, if using. Add flour mixture, and combine on low speed, scraping with spatula if necessary, until flour is just incorporated and dough sticks together when squeezed with fingers.
Pat dough into prepared pan. If rolling out dough to cut into shapes, form into a flat disk; wrap in plastic. Chill until firm, at least 1 hour.
Roll basic dough 1/4 inch thick; chill. Roll chocolate dough (see Chocolate Wedges recipe) 1/4 inch thick. Place basic dough on top of chocolate dough. Cut into 1 1/2-by-3-inch triangles; chill. Heat oven to 325 degrees. Bake until firm and just starting to color, 20 to 25 minutes.