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