In a large bowl, cream together the butter and sugar. Beat in the eggs one at a time, then stir in the vanilla. Combine the flour, cocoa, baking soda and salt, gradually stir into the creamed mixture. Fold in chocolate chips. Drop by rounded spoonfuls onto unprepared cookie sheets.
Bake for 8 to 10 minutes in the preheated oven. Do not overbake. Allow cookies to cool on baking sheet for 5 minutes before removing to a wire rack to cool completely.