If you want to make ganache frosting and you do not have heavy cream on hand you can substitute melted ice cream. Melt the ice cream and then measure out the same amount that was called for in heavy cream.
Chop chocolate into small pieces so that it will melt faster. Place in a medium heatproof bowl.
Place the whipping cream (or melted ice cream) and butter in a small saucepan; heat just to a boil.
Pour over the chocolate pieces and allow the mixture to stand for a couple of minutes; then slowly stir until smooth. Be sure to stir mixture gently so that you do not cause air bubbles to form in the ganache.
Place the cake, bottom side up, on a wire rack over a cookie sheet. Brush off excess crumbs from the cake.
Spread a thin layer over the top and sides of the cake to provide a crumb coating. Place the cake in the refrigerator for 5 minutes for crumb coat to set.
Then begin adding the ganache to the top of the cake and with a long metal spatula spread it out towards the edge of the cake. Allow it to run down the sides and then spread it out so it covers the sides evenly. Allow excess to drip unto the cookie sheet.
Pour more ganache on the top as needed until the sides are coated and the top is covered and smoothed out evenly. Place in the refrigerator for about 10 nminutes to allow ganache to set up.