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. Sift together the flour, cocoa, bicarbonate of soda and salt and stir into the creamed mixture. Fold in the white chocolate chips.
Drop by rounded teaspoonfuls on to ungreased baking trays and bake for 8 to 10 minutes, or until cookies are set. Allow cookies to cool on baking sheet for 5 minutes before removing to a wire rack to cool completely.