Melt chocolate in a bowl set over a saucepan of simmering water. Let cool slightly. Beat together butter, sugar, and salt with a mixer until pale and fluffy, about 5 minutes. Gradually beat in chocolate, then sour cream, and beat until thoroughly incorporated. Frosting should be spreadable. If too loose, refrigerate, stirring occasionally, until thickened. Use immediately, or transfer to an airtight container and refrigerate up to 3 days. Before using, bring to room temperature, and beat until smooth.