Cream butter or margarine and sugar in large mixer bowl. Add eggs and vanilla, blend well. Combine flour, cocoa, baking soda, and salt, gradually blend into creamed mixture. Stir in nut, if desired. Drop by teaspoonfuls onto ungreased cookie sheet. Bake at 350 degrees for 8 to 9 minutes. (Do not over bake. Cookies will be soft. They will puff during baking, flatten upon cooling.) Cool on cookie sheet until set, about 1 minute. Remove to wire rack to cool completely. Makes about 4-1/2 dozen.