I made my first batch of Italian buttercream a month ago. It turned out beautiful. Today I tried to do it again, double the recipe (10 egg whites), like yours, mine turned into a soupy mess when I added the butter. I whipped and whipped and whipped... still a soupy mess and now it seems to have curdled. As suggested bellow, I am resting the buttercream in the fridge and will try to whip it later. keeping my fingers X that it will 'solidify'.