I have a cake I'm doing for a baby shower. The cake is due at 1pm on Saturday.

I'm going to be covering baby blocks in fondant but here's the kicker...

I'm using a buttercream that is completely butter and not shortening. I would like to assemble the blocks tonight but do not want to refridgerate them with the fondant because I've always had bad results from refridgerated fondant.

Will my buttercream go bad if I cover the blocks tonight and let them sit out overnight?

your cake will be fine ....I use "all butter" butter cream under my fondant it last great.......it it just a plain frosted cake over night will be fine .....as long as your house is not too hot......

Personally, I would do it and it would be fine. However, state health laws do vary. Salted butter has a longer life outside the fridge then unsalted, and all the sugar will keep it stable. Alot of people keep their butter sitting out all the time, people used to do it all the time in the old days before refrigerators!