Heat oven to 350°F. Grease a baking sheet.
In food processor, combine flour, cocoa and sugar. Turn machine on; slowly add butter, 2 tablespoons at a time, processing until incorporated. Add vanilla; process until dough forms ball on top of blades.
Between palms of hands, roll pieces of dough into balls about 1/2 inch thick. Place on prepared baking sheet. With floured bottom of drinking glass, press down dough to 1/4-inch thickness. Sprinkle dough with granulated sugar and pecans; press again to flatten.
Bake in 350°F oven 10 minutes, until firm. Let cool on baking sheet on rack. Store airtight at room temperature up to 1 week.