In a large bowl, cream butter and sugar until light and fluffy, 5-7 minutes. Beat in eggs and vanilla. Combine flour, cocoa, baking soda and salt; gradually add to creamed mixture and mix well. Stir in chocolate chips.
Drop by rounded tablepoonfuls 2 in. apart onto greased baking sheets. Bake at 350° for 8-10 minutes or until set. Cool for 2 minutes before removing from pans to wire racks.