I am used to make MMF to cover my cakes. Will MMF hold up to the heat in the south. We are having 100 degree temperatures here now. Also what would be the must butter cream recipe to use?

Yes, MMF will hold up to the heat but your buttercream under it may not. The outside will be soft but not melted but the inside buttercream will melt. Someone was telling me to use ganache under fondant when it's that hot. Obviously, if someone is serving cake this time of year, then hopefully they have a fridge nearby to keep the cake cold till service. There is also a recipe for buttercream here that is supposed to be great in the heat as well.