Recipe adapted from Bon Appétit
1 1/2 cups all purpose flour
1/2 cup sugar
1/2 teaspoon salt
3/4 cup (1 1/2 sticks) chilled unsalted butter, cut into 1/2-inch cubes
1/2 cup natural unsalted pistachios, lightly toasted, chopped
1 large egg yolk
3/4 teaspoon vanilla extract
Mix flour, sugar, and salt in processor. Add butter, pistachios, egg yolk, and vanilla. Pulse until moist ball forms. Transfer dough to work surface. Divide dough in half. Form each dough half into 8 x 1 1/4-inch log (if dough is too soft, chill until firm, about 30 minutes). Wrap logs in plastic; refrigerate until firm, about 4 hours. Can be made 5 days ahead; keep chilled. Preheat oven to 325°F. Slice logs into 1/4-inch-thick rounds, rolling log after every few slices to retain round shape. Place rounds on ungreased baking sheets, spacing 1 inch apart. Bake shortbread until barely golden, 16 to 18 minutes. Cool shortbread on baking sheets.
Optional: Melt 3 ounces bittersweet and 3 ounces semisweet chocolate, stir and dip cooled cookies halfway into chocolate. Finely chop 1/4 cup of pistachios and sprinkle over cookies. Let set for about an hour until chocolate hardens.