I used the White Chocolate Mouselline Buttercream from TCB. I do take out a small amount of the buttercream and add the melted white chocolate to it, then add it to the rest of the buttercream. I’ve lost some white chocolate (and a whole batch of buttercream) when I added it directly to the buttercream (I can’t remember if the white chocolate hardened into lumps or if it curdled), but as per the great people on the forum, I learned that little trick which has prevented me from ruining another batch! Also, I added some Navan liqueur—not as much as the recipe calls for, but about 1/2.