I need some help with icing for a cake?

I want to make a chocolate cake and have a peanut butter marshmallow icing I mixed the 2 together to see the consistency, it tastes good but seems a little to thick for a cake. Also it seems like if I ice the cake with just the peanut butter marshmallow concoction it will be disastrous, I need to thin it down a little what should I use?