1. Place chocolate in a medium sized bowl.
2. Heat cream over medium low heat until it just barely begins to boil. Remove from heat immediately. Do not over-heat!
3. Pour hot cream over chocolate and let it sit for 1 minute. Gently stir mixture until chocolate is melted and blended with the cream. Add in the butter and combine. Do not beat the mixture, you want to minimize the introduction of air bubbles. 
4. If used as a glaze: press a piece of plastic wrap against the surface and let cool slightly--it must be pourable, but not hot. If whipped for a frosting or for truffles: place a piece of plastic wrap covering the surface of the chocolate mixture and refrigerate or let sit at room temperature until thickened. Whip with an electric mixer.

Storage of ganache