Heat oven 350.
In large mixer bowl, beat butter and sugar until light and fluffy. Add eggs and vanilla; beat well. Stir together flour, cocoa, baking soda and salt; gradually blend into butter mixture. Stir in chips. Drop by rounded teaspoonfuls onto ungreased cookie sheet.
Bake 8-9 minutes. (Do not over bake; cookies should be soft. They will puff while baking and flatten while cooling). Cool slightly; remove from cookie sheet to wire rack. Cool completely.