I am currently using the easy SMBC recipe (Liz Marek). I refrigerate my cakes before pick up/delivery but as the hot summer months approach, would it be wise to switch to 50% butter and 50% shortening to make this more stable? Would it work? I've also read adding a bit of cornstarch to some buttercreams makes them more stable? Any advise?
I could not really answer your question because I've never made Liz Marek's easy SMBC. I've made SMBC with all butter, but never used 50% shortening. I've made American buttercream using all butter, half and half, and all shortening, which is the most stable in heat. The cornstarch trick, I've heard of but never tried.
You can sub up to half shortening without any trouble. It's just fat, minus the water that's in butter. I wouldn't go more than that because of the flavor.
I've never heard of cornstarch in this type of buttercream, but I do add a small about of powdered sugar too my SMBC. I don't think the tiny bit of cornstarch in it really effects stability though.
Good advice from rychevamp. And the addition of cornstarch does work, as I have tried it.