re: White chocolate buttercream filling

I think everyone’s got it covered, but I would imagine it’s like making chocolate SMBC or IMBC except white chocolate instead of dark melted chocolate. I’d also adjust down the sugar in the base buttercream though since there’s so much more in white and less actual chocolate to balance out the sweetness. I’ve also heard Rose Levy Beranbaum has an amazing white chocolate CREAM CHEESE frosting that’s much more stable than regular cream cheese frostings. Not sure if your client wants cream cheese flavor, but that might be another option.