Preheat the oven to 350 degrees. Beat the butter and the sugars together in a large mixer bowl until well blended. Beat in the eggs and vanilla. Stir the flour, cocoa, baking soda and salt together. Gradually add to the butter mixture, beating until thoroughly blended. Stir in the chips. Drop by rounded teaspoons onto an ungreased cookie sheet. Bake until set (9-11 minutes). Cool slightly before removing to a cooling rack.