Preheat oven to 350 degrees. In small bowl, combine flour, cocoa, baking soda and salt; set aside. In large mixer bowl, beat butter, sugar, brown sugar and vanilla extract until creamy. Add eggs one at a time, beating well after each addition. Gradually add flour mixture. STir in chocolate pieces. Drop by rounded teaspoonfuls onto ungreased cookie sheets. Bake 9-10 minutes until edges are lightly browned. Let stand 2 minutes before removing from cookie sheets; cool completely.