In a large bowl, cream together the butter and sugar until smooth.
Beat in the eggs one at a time, then stir in the vanilla.
Combine the flour, cocoa, baking soda, and salt; stir into the creamed mixture until just blended.
Drop by rounded spoonfuls onto ungreased cookie sheets.
Bake for 8 to 10 minutes in the preheated oven, or until set. Allow cookies to cool on baking sheet for 5 minutes before removing to a wire rack to cool completely.