Preheat oven to 180 degrees C. In a bowl, sift together flour, cocoa, bicarb soda and salt.
Beat butter and sugars until creamy. Add eggs and vanilla; beat well. Add combined flour, cocoa powder, bicarb soda and salt; mix well. Stir in oats and raisins and mix well.
Drop dough by rounded tablespoonfuls onto ungreased baking trays. Bake 10 to 12 minutes until biscuits are almost set. Do not overbake. Cool 1 minute on baking trays, then move to wire racks. Store tightly covered.