In large bowl, beat butter, sugar, eggs, and vanilla until light and fluffy. Combine the flour, cocoa, baking soda, and salt; stir into the butter mixture until well blended. Mix in the chocolate chips . Drop by rounded teaspoonfuls onto ungreased cookie sheets.
Bake for 8 to 10 minutes in the preheated oven, or just until set. Cool slightly on the cookie sheets before transferring to wire racks to cool completely.