Chocolate Streusel Bars

Preheat oven to 350 °F (175°C) . Lightly grease 13 x 9 x 2-inch baking pan; set aside. In large mixing bowl, combine flour, sugar and cocoa; mix well. Cut in butter until crumbly (mixture will be dry). Reserve 2 cups crumb mixture. Press remaining crumb mixture onto bottom of prepared pan. Bake for 15 minutes. Meanwhile, in large mixing bowl, beat cream cheese until fluffy. Gradually beat in sweetened condensed milk until smooth. Add egg and vanilla; mix well. Pour evenly over baked crust. Combine reserved crumb mixture with nuts, if desired; sprinkle evenly over cheese mixture. Bake for 25 minutes or until bubbly. Cool in pan on wire rack. Chill. Cut into bars. Store covered in refrigerator.