Just made my first recipe of Rose’s Mousseline Buttercream. I did have a bit of problem with the syrup. I left it on low heat on my gas stove while I beat the egg whites, but it just kept bubbling vigorously and I was afraid it would over-cook. So I took it off the heat while I beat the whites. There was a whole lot of crystallized sugar around the edges when I came back to the syrup. I made the mistake of scraping the edges and stirring it into the syrup—result, even more crystals. I beat it into the egg whites anyway and oddly enough it all ended up just fine!
Next time I think I will take the syrup off heat right away—and not scrape down any crystals.