Heat oven to 350F. In large mixer bowl, beat butter, sugar, eggs and vanilla until light and fluffy. Stir together flour, cocoa, baking soda and salt; add to butter mixture. Stir in chocolate chips and nuts, if desired. Drop by rounded teaspoonfuls onto ungreased cookie sheets. Bake 8 to 10 minutes or just until set. Cool slightly; remove from cookie sheet to wire rack. Cool completely. Makes about 4 1/2 dozen cookies.