In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Sift together the flour, cocoa, baking soda, and salt; stir into the creamed mixture.
Mix in the chocolate chips. Drop dough by teaspoonfuls onto ungreased cookie sheets.
Bake 8 to 9 minutes in the preheated oven. Cookies will be soft. Cool slightly on cookie sheet; remove from sheet onto wire rack to cool completely.