Just needs warming up a touch to get it back to the emulsified stage. Stick it over a double boiler for a little bit and re whip or do as previous posters suggested and temper it a bit with a third warmed.
Depends what sort of chocolate cake you want really.
There's devil's food cake, australian style mud cake, chocolate sponge cake, flourless chocolate cake, chocolate genoise to name just a few. All have different tastes and textures.
What are you looking for?
The most important thing about getting smooth fondant is a smooth undercoat. I use ganache and I get it a smooth as possible using the upside down method then I use sugarpaste direct fondant. To smooth it I use clear acrylic smoothers so I can see what I'm doing and then use two pieces of acetate to sharpen the edges.
Just to give you an idea... This is with swiss meringue buttercream. You can see how smooth it is before fondant.
I use sweet appolitas recipe, works every time. When you add the vanilla at the end it will look like it is not incorporating properly (because you're pouring liquid into fat) but it'll get there eventually.