Prepare a double boiler. If you don't have one, you can use a small saucepan and a heatproof bowl. Fill water into the saucepan (about a quarter of the saucepan high).
Place the heat proof bowl on the saucepan. Make sure the bowl is larger than the saucepan so that it sits on the pan and not inside. The bowl should not be touching the water underneath.
Add chocolate into the bowl. If using a chocolate bar, chop the chocolate into small, uniform pieces for easy melting. If using chocolate buttons or melts, no need to chop them.
Turn on the heat to a medium flame and let the stim from the water heat up the bowl and the chocolate. Stir to expedite the melting.
As soon as the chocolate is almost completely melted, add the butter. Again, for quicky melting, cut the butter into small cubes.
Remove the chocolate mixture from the heat immediately after adding the butter and let the butter melt in the heat of the chocolate. Stir to mix. Let the chocolate mixture cool down to room temperature.
In a separate bowl, measure and sift the powdered sugar.
Pour the cooled chocolate mixture into the powdered sugar.
Add the vanilla extract.
Mix until the sugar is well combined and the chocolate icing comes together.
If the icing is too stiff, add some cream to it and beat it until smooth. Add the cream in small increments of one tablespoon at a time to avoid the icing from becoming too thin.
The icing can be used immediately.