Chop the chocolate into small pieces (if using a chocolate block). Make sure the knife and the cutting board, as well as your hands are completely dry as any traces of water in the chocolate can cause it to seize when double boiled. To double boil the chocolate, you can use a double boiler. If you don't have one, you can easily use a small saucepan and a slightly larger heat proof bowl. Fill some water in the pot and then place the heat proof bowl over it. The bowl should rest on the saucepan but should not be touching the water. The chocolate is melted by the heat from the water steam and not the hot water. Once you have set up your double boiler, turn on the heat to medium heat. Add chocolate into the prepared bowl.
Let the chocolate sit in the bowl without stirring for a minute or 2. Stir occasionally after that to expedite the melting.
Once the chocolate is almost fully melted, add in the butter and remove the mixture from heat by lifting the bowl off the saucepan. Melt butter from the heat of the chocolate. For quick melting, cut the butter into small pieces and make sure it is at room temperature.
Stir to mix the chocolate and the melted butter.
In a separate mixing bowl, sift the powdered sugar.
Once the chocolate mixture has cooled down to room temperature (which it will almost immediately after mixing in the butter), pour it into the powdered sugar.
Mix well until all the ingredients are well combined.
If the icing appears too dry, add some heavy cream (or milk) and mix well before using. The frosting can be used immediately.
This recipe makes approximately 990g of chocolate frosting (3 ⅓ cups). It is sufficient to generously fill and frost a 2 layered 8-inch or 9-inch round cake.