Stir together white chocolate and 1/2 cup boiling water until chocolate melts. Cool; chill 30 minutes.
Beat chilled white chocolate mixture and butter at low speed with an electric mixer until blended. Beat at high speed 2 to 3 minutes or until fluffy. Gradually add powdered sugar and salt, beating at low speed until blended. Beat at high speed until smooth and fluffy.
Note: For testing purposes only, we used Ghirardelli white chocolate.