Heat oven to 375 F.
Sift flour, sugar and salt into bowl. Using fork, thoroughly mix in butter, egg yolk, milk and vanilla.
Form into walnut-sized balls. Place on ungreased baking sheet.
Using bottom of greased glass dipped in sugar, press balls to 1/16-inch thick. Brush with slightly beaten egg white. Sprinkle with pistachios, sumac and black pepper.
Bake 8-10 minutes.