Melt chocolate in heatproof bowl set over pan of simmering water, stirring occasionally. Let cool slightly.
Sift sugar, cocoa, and salt into a large bowl. Put cream cheese and butter into mixer bowl; mix on medium until fluffy. Reduce speed to low. Add sugar mixture; mix until combined. Mix in melted chocolate and sour cream. Use immediately, or refrigerate airtight up to 2 days; bring to room temperature, and stir before using.