Preheat oven to 375°F. In saucepan, melt butter with chocolate until smooth. Blend sugar into the chocolate. Beat egg with vanilla and stir into chocolate. Add flour, baking soda, and salt. Drop tablespoons of dough 2" apart on baking sheets. Dip a drinking glass into additional sugar and flatten cookies slightly. Bake for 15 minutes or until edges are crisp. Transfer to cooling rack.