Slowly melt butter and unsweetened chocolate. Meanwhile blend sugar, eggs, buttermilk, and vanilla. Beat until smooth. Sift flour, salt and baking soda together. Add to egg mixture to chocolate mixture and blend well. Add flour mixture and beat until smooth. Fold in choclate chips. Grease muffin tins well. Fill each cup 2/3 full with batter. Bake at 375 deg for 20 to 25 minutes. These tend to be habit forming when served with cold milk.