Semi Sweet Chocolate, chopped - 3/4 cup
Heavy Cream - 1/2 cup
Butter, unsalted - 2 tbsp
Instead of adding the chocolate to the cream mix when it's still on the stove, you can remove and add. I have done both ways and have not found any difference.
Also when you add the ganache to the cake when it is still hot, you will find it as only a sauce and not a thick cream. Allow it to cool and then you will find that it become a nice thick layer over the chocolate cake.