In a microwave safe bowl heat the chopped white chocolate 30 seconds at a time until it's mostly melted. Stir the white chocolate until it's smooth. If you have any lumps heat for another 30 seconds and stir again.

In the bowl of your mixer add the whipping cream, butter and vanilla. Beat until combined. Add the white chocolate to this mixture and beat until smooth. 

Add the sifted powdered sugar 1 cup at a time and beat until a smooth frosting forms. 

I like this consistency for frosting my cake, but if you are going to pipe the frosting onto the cake you may want to add more sifted powdered sugar for a stiffer frosting.

Store any unused frosting in the refrigerator for up to 7 days.