I make a lot of vegan cakes which use oil instead of butter so they remain really soft and spongy even when in the fridge. However, I recently made a cake that called for creaming butter with sugar, etc and when it came out it was super soft and spongy, but once I put it in the fridge it became pretty solid. Is that because the butter in the cake hardens somewhat when cold but when the cake comes to room temperature again it will be soft?

Any tips or advice would be much appreciated! 

Also, if it is the case that because it is a butter-based cake that hardens when in the fridge--how would you go about decorating with such a cake? I always frost a chilled cake and bake ahead of time and store in the fridge. How would you do that with this type of cake?