In a large bowl, cream together the butter and sugar until smooth. Beat in the eggs one at a time, then stir in the vanilla. Combine the flour, cocoa, baking soda and salt; stir into the creamed mixture. Fold in the white chocolate chips. Drop by rounded teaspoonfuls onto ungreased cookie sheets.
Bake for 8 to 10 minutes in the preheated oven, until cookies are set. Allow cookies to cool on baking sheet for 5 minutes before removing to a wire rack to cool completely.