In a double boiler over medium-low heat, melt white chocolate, stirring to ensure all bits are incorporated. When smooth and glossy, leave on very low heat until ready to mix into frosting.
Using a hand or stand mixer equipped with a whisk, cream together butter and cream cheese. Add salt and vanilla and mix well.
Gradually add melted chocolate and mix well, scraping sides of bowl.
With mixer on low speed, add powdered sugar 1/4 cup at a time, scraping sides.
Chill for 2 hours before spreading or piping.
Keyword cakes, cream cheese, cupcakes, frosting, white chocolate