Ok so i am just paranoid and double checking with this....so i am fine with leaving a BC cake covered in fondant out and not in the fridge but if i left just a purely buttercream covered cake out overnight in a covered glass cake dish it's still good to eat isn't it?

It's just the buttercream made from butter and icing sugar and the recipe says it shouldn't be refridgerated but i am not sure if that means only for the day or can be overnight too

It's fine to set out for a fews days actually. (I know, this sounds wrong, but it really is okay) The only time I have heard of a cake spoiling (molding) was from it being in a cake carrier. Although, once it's cut, I prefer to refridgerate.

If you are using a fruit filling, however, it will have to be refrigerated regardless of what the cake is covered with or it will spoil. Not the buttercream, the filling.

Our moms and our grammas left cakes out all the time and we all survived! If the recipe says it's ok to leave it out, then it's ok to leave it out.

thanks for the reassurance lol
And yeh i know the rule for certain fillings, this is just a simple buttercream for outside as well as the filling