Melt the white chocolate in a heatproof bowl set on top of a saucepan filled with two inches of simmering water. Stir occasionally, and remove from the heat before it is completely melted. Stir until smooth. Allow to cool until almost starting to set, then stir in sour cream. It is now ready to use. For a fluffier frosting, allow to cool completely then beat with an electric mixer.
