Whipping heavy cream with melted chocolate
If you are trying to make chocolate frosting using whipped cream, you need to: 



Whip the cream first. 
Melt the chocolate and add some amount of whipped cream to the melted chocolate (mix it by cut and fold method)
Add this mixture to the remaining whipped cream and fold it. Don't
over-mix it, it would knock out the air from the whipped cream.


To make chocolate ganache frosting, you just need to follow simple steps:


The ratio to make chocolate ganache frosting I use is 2 parts of chocolate to one part cream (by volume). 


Heat the cream and bring it to the boil. 
Once cream comes to boil, add the hot cream to chocolate. Let it sit for2-3 minutes. Now stir till it is well combined.
Let it cool completely. It would thicken up. Use it as your requirement.


If you feel that the ganache is too thick, you can add hot milk and mix it and then cool.

The images have been taken from internet.