I use a different method than DenLach uses.
While the white chocolate is still lukewarm but not solid, I put in a stand mixer bowl and put on low speed.
While this is going, I add a bit of milk or cream before it starts to harden up, to keep it from hardening and keep it where it can be whipped. Depending on the recipe, usually about one or two tablespoons.
Once the liquid has been mixed into the chocolate, then I add the butter, lard, or shortening....whatever you are using to make the buttercream, and a 1/4 teaspoon of salt. I then put the mixer on medium speed for about a minute, then turn it back to low.
I then start sifting in my sugar slowly. If you do it too fast, it will clump up. Once you have your sugar in there, and it has incorporated pretty well, turn the mixer to medium speed again. You can put in your flavorings if you have any, at this time.
Depending on the sugar level you want, I usually put a tablespoon of apple cider vinegar in there at this point, to keep that hard sweetness level down a bit.
After about two minutes on medium, you can then put it on high and whip it until it gets to the point you want it.
Once that happens, I remove the frosting, put it in an airtight container, use a water spray bottle and give the top of the frosting a fine mist, then I place some plastic wrap directly on the frosting and pat it down into the container until its even and doesn't have air bubbles under the wrap. Then I put the airtight lid on, and make sure the air has been pressed out.
I then put the frosting in the fridge over night.
The next day I take it out and let it get to room temperature. I then use a hand mixer on low to mix up the frosting.
If the frosting is too thick, I add some more liquid...it can be more flavoring, water, milk, cream, or even sour cream. Probably about a tablespoon.
Once the frosting has been "loosened", its ready to use.
I find that making something like this the day before seems to work better after being stored over night in the fridge. I've used fresh white choc buttercream and have had difficulties with using it for decorating, but the next day, it just seems perfect.