Place the chopped white baking chocolate in a heatproof bowl set over a pot of simmering (not boiling) water, making sure the bowl does not touch the water. Stir occasionally until melted and smooth. Alternatively, melt the chocolate in the microwave in 15-second intervals, stirring after each.
Let the melted chocolate cool to room temperature for about 15 minutes before using. It should be smooth and fluid but not warm enough to melt the butter.
In a large mixing bowl, beat the room-temperature butter with an electric mixer on medium speed until creamy, about 2 minutes. Reduce the mixer speed to low and gradually add the powdered sugar, about 1/2 cup at a time, beating until fully incorporated and smooth. Scrape down the sides of the bowl as needed.
Once the white chocolate has cooled but is still pourable, slowly add it to the butter mixture while beating on medium speed. Continue beating until the frosting is smooth and well blended, about 2 minutes.
Beat in the heavy cream, vanilla extract, and salt on medium speed until light and fluffy, about 1 minute. Adjust the consistency as needed—add more cream, 1 tablespoon at a time, for a softer texture, or a pinch more salt if the frosting tastes overly sweet.
Use the frosting immediately, or cover and refrigerate.