I had a few disasters with Buttercream today. Used it cold from the fridge then it curdled guessing it has to be really soft?

Anyway, I thought I'd be better with shortening. Easier to work with? I'm confused which to use. I've heard of frymaster, anyone used it before with any success? Can you put the iced cake in the fridge or leave out?

Thanks, I'm so clueless! DH is hassling me that I should've outsourced it