If using in the heat of the summer, you can substitute 1 stick of butter for shortening that can hold up to better in hotter temperatures. I personally try to avoid using traditional shortening. You can try using other better for your shortenings that are available. In the hot months I also like to add about 2tbl meringue powder per batch, once the buttercream is all whipped and made, ready to go I mix in meringue powder to stabilize it little more.
Remember butter starts to melt at 82.4F -96.8F (26-36C), so if you are planning to make this buttercream in the heat of the Summer, consider cake, or cupcake display inside, in the air conditioned area.
Swiss Buttercream Flavors