A chocolate version of our Basic Shortbread Wedges.
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.
Sift 1/2 cup cocoa with flour and salt. Press dough into 8-inch springform, or roll it to 1/4 inch thick; cut into wedges. Score with a knife; chill. Heat oven to 325 degrees. Bake until firm, 20 to 25 minutes.