In a large bowl, cream together the butter and sugar. Beat in the eggs one at a time, then stir in the vanilla. Combine the flour, cocoa, bicarb soda and salt, then gradually stir into the creamed mixture. Fold in chocolate chips. Drop by rounded spoonfuls onto unprepared baking trays.
Bake for 8 to 10 minutes in the preheated oven. Do not overbake. Allow biscuits to cool on baking tray for 5 minutes before removing to a wire rack to cool completely.