I really love Swiss butter cream icing, it is so smooth and light, but it does not hold up in extreme weather. There is this recipe from Woodland Bakery in New Jersey that is to die for. They have this you tube video for it that calls for shortening to make it more stable. You should give it a try, you won't be disappointed. I'll give you the recipe.
Fresh Large Egg Whites 6 Large
White Sugar 12oz (1 1/2 cups)
Confectioners Sugar 8oz (2cups)
Shortening 6oz (3/4 cup)
Butter 1lb (2cups)
Vanilla Extract 1T
**I forgot to add the vanilla extract in the video. You simply add it last.
***THIS RECIPE CAN BE STORED 4 days room temp, 2 weeks fridge, 2 months freezer
**its fine to store it in the fridge/freeze, but just know that you will now have to bring it back to room temperature. Take it out HOURS before you need it, and let it get to room temp, then you will put it back on the mixer and with the PADDLE attachment on low- medium and let it mix, it will look as if it is BROKEN, and curdled, don't be alarmed this is NATURAL, it will come back together eventually, you can turn the speed up to med-high and it will mix out smooth
If you go on you tube and put crumb boss in the search menu and then scroll down to Swiss butter cream recipe, she will walk you through it.