ECSTATIC to report back that my second and third trials of the Mousseline Buttercream have stood up!
I made a touch more syrup (using about 15g more sugar) to make up for what is left clinging to the saucepan and the glass measure that I am unable to scrape out. I reduced the sugar that is beaten together with the egg whites by the same amount to account for the extra sugar syrup, since I can’t be sure exactly how much will finally make its way into the buttercream! And, a less sweet buttercream is always a benefit to my taste.
I halved the amount of alcohol this time. And kept the passionfruit puree at about 2/3 of that called for (I tried using only half but did not find the flavour coming through as strongly so I added a bit more). I also kept the buttercream sitting in a bowl of iced water to keep cool while filling and frosting the cake, and froze the cake briefly between crumb coating and final coating - all of which I SHOULD have done with the first test cake, but was too lazy to do.
I left the cake that I frosted out this morning at room temp (about 82-84 deg F) for about 2 and a bit hours, and it held up fine. No melting, and I could still slice off a piece cleanly. It just goes to show when dealing with hot weather, all the additional steps may be a pain but crucial.
Now, the only thing is, I didn’t find the Downy Yellow Buttercake a good match for this passionfruit mousseline buttercream because it masked the subtle passionfruit flavour a bit too much (I think the buttercake has its own strong flavour which I love on its own), so will probably go back to the lighter (and, I guess, blander) sponge that I had used the first time. Yes, for once, I love the buttercream more than the cake!
Thank you for all the advice, everyone!
If the cake turns out OK on Monday, I will post pics.