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.
Drop by teaspoonfuls onto ungreased cookie sheet.
Bake at 350 degrees for 8 to 9 minutes.
Cookies will be soft.
They will puff during baking, use the ball of a spoon to gently flatten as they cool.
Let sit on cookie sheet until set,about 1 minute.
Remove to wire rack to cool completely.