1. Melt chocolate and butter in a 2-quart glass bowl in microwave oven (about 1 minute at HIGH). Stir.
2. Add 1 1/2 cups confectioners' sugar, half-and-half and salt; beat until smooth. Cook uncovered in microwave oven 1 1/2 minutes at HIGH, or until bubbly.
3. Add vanilla extract, then remaining sugar in thirds, beating after each addition until smooth.