Coarsely chop chocolate. A serrated knife is best for the job; its sawlike teeth grab the chocolate, breaking it up.
Bring cream just to a boil over medium-high heat. Pour over chocolate, and add salt. Let stand for 10 minutes (don't stir -- doing so will cool the ganache too quickly, making it grainy).
Stir with a whisk until smooth and shiny to break up any pieces and emulsify cream and chocolate.
Chocolate will often settle on the bottom or sides of the bowl. Scrape dish with a rubber spatula to incorporate all of it.
To make a whipped filling or frosting: Let ganache cool to room temperature, stirring often, 45 minutes to 1 hour. Beat it with a mixer on medium-high speed until paler and fluffy, 2 to 4 minutes. (This will yield about 2 cups.) Transfer to a disposable pastry bag with a large opening and use it as a filling for mini whoopie pies or spread it over cupcakes.