Heat the chocolate in a double boiler or medium heatproof bowl set over (not in) a saucepan of simmering water, stirring often, until melted and smooth. Remove from heat and let cool, 10 to 15 minutes.
Using an electric mixer, beat the butter on high until light and fluffy, 3 to 5 minutes. Reduce mixer speed to low. Gradually add the sugar and beat until smooth, scraping down the sides of the bowl as necessary. Add the chocolate, sour cream, and salt and beat to combine.
The frosting will keep in the refrigerator for up to 1 week. Bring to room temperature and, using an electric mixer, beat on low until light and fluffy before using.
Makes 5 cups