In food processor, blend flour, sugar, cocoa and 6 T butter until mixture resembles coarse meal; press into 8" square pan. Bake at 350°F for 15 minutes. In heavy saucepan, boil syrup to 240°F on candy thermometer; remove from heat and stir in remaining 2 T of butter. Whisk eggs with vanilla and add syrup in slow stream, whisking until well blended. Stir in pecans; pour over chocolate crust. Bake at 375°F for 20 minutes. Cut into 2" squares when cool.