I am making a wedding cake in July. It will be a black and white cake. What will work better with the heat:Fondant or buttercream? Help
Shortning based buttercream most likely.
Is the reception indoors with air conditioning or outdoors? If it is outdoors, I would either use a shortening based buttercream or fondant. Either should be fine,as long as the cake is not in direct sunlight. If it is going to be indoors with a/c and you are just worried about transport, you could do a buttercream that is 50/50 shortnening and butter. I would steer clear of a 100% buttercream (i.e. SMBC).
Yesterday, I delivered 9 dozen cupcakes to a wedding reception in 90 + degree heat. I used toneedna's buttercream (50% butter and 50% hi ratio shortening). About 20 minutes prior to delivery, I started my car and cranked up the ac. That way, when I put the cupcakes in my SUV, the interior was nice and cold. This worked beautifully, and the cupcakes stayed in perfect condition. HTH!
As cakegirl said, b'cream should be fine. I made 1,000s of cakes in CA - triple digit heat, no humidity - all iced in b'cream and never had but one melt.