I live in Australia and had to do a two tier princess/pirate themed buttercream cake on a 44 degree summer day. It also had gumpaste decorations attached. I was having nightmares it would melt and the decorations would slide off but the recipe I used worked great. I used a Swiss meringue buttercream recipe using 50/50 butter and Crisco and added a cup of sifted pure icing sugar at the end of mixing. I used pasteurized egg whites. There is a free tutorial on Craftsy called Modern Buttercream which I used as a guide recipe adjusting butter/shortening as needed. The gumpaste decorations were premade and dried over foam dummy cakes. The whole cake was assembled the day before, decorated and kept in the fridge until delivery when it then travelled 2 hrs to the party in an air conditioned car. I was never so relieved as when I had delivered the cake and the feedback was so positive. I am no longer afraid of buttercream in summer though I am yet to bight the bullet and use it under fondant.
