Melt chocolates in a double boiler or a large metal bowl set over a saucepan of simmering water, stirring occasionally. Remove bowl from heat, then whisk in sour cream and vanilla. Cool to room temperature, stirring occasionally (frosting will become thick enough to spread). You must work quickly and spread the frosting before it becomes too thick. (If icing does become stiff, reheat over simmering water, then cool and try again.)
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