Very easy and so completely delicious that you won't believe it! Adapted from a recipe in the Washington Post that was adapted from a recipe by Rose Levy Beranbaum that was posted over at the King Arthur llour website. (Lots of steps here.) I've upped the ratio of white chocolate to cream cheese and butter (as is only right) and eliminated some fiddly measurements and one ingredient. My version is just poi-fect. Note what I say in the recipe about using unsalted butter. This may be one of the few times when making the effort to buy this ingredient is justified.
Melt the chocolate by putting it in a microwave-safe container and heating for 2 minutes at 50% power. It will be only partially melted; stir until smooth, heating for an additional 30 seconds at 50% power if needed. Why any recipe ever tells you to microwave chocolate on high is a mystery to me. Let the chocolate cool until cool. Don't neglect this step! You don't want to melt your butter. But don't let it cool so long that it starts solidifying around the edges and you end up with little chunks of chocolate. If that does happen, of course, just re-microwave it at 50% for maybe a minute. And then you may have to re-cool it. So don't let it sit too long in the first place!
Meanwhile, use your stand mixer with the whisk attachment (preferred) or a hand mixer to whip the softened butter and cream cheese together. Add the vanilla, then gradually mix in the melted white chocolate. Whip until creamy and thick.
*You'll read in many recipes that you shouldn't use chocolate chips to replace baking chocolate, but that's simply not true, especially for a frosting recipe when you want a certain amount of stiffness. Just be sure that any chocolate that you use, chips or bars, is made with cocoa butter and not just palm oil or hydrogenated vegetable oil. I've used my local grocery chain's brand, Kroger's Private Selection, with great success.
**Because the cream cheese is salted, I feel that the use of salted butter makes the final product a little too salty. You certainly don't have to do this, and if you're making the frosting and realize that you don't have anything but salted butter, please just go ahead. I think you'll be happier with the end result if you remember to get the unsalted version when you're out grocery shopping. I say this as someone who has always refused to bother with unsalted butter even though pretty much every recipe I have says to use it. I typically cut the amount of salt down in recipes that call for unsalted butter, but for this recipe there's no additional salt for me to cut. Your call!