How does it hold up?????
depends how hot. you minght want to add some shortening in place of some butter, gives it a bit more stability.
I had the same question a few weeks back. I had a cake that was going to be sitting outside for 6 hours. I used indydebi's buttercream recipe, and it held up great.
And just in case you don't know, dream whip is in the grocery store by the pudding mix. Hope this helps!