Ganache must be cool to be whipped properly. Overwhipping will make the mixture grainy or too stiff. If this happens, remelt, chill, and rewhip mixture.
Place chocolate in large heat-proof bowl. Bring cream to boil in medium-size saucepan over medium-high heat; pour directly over chocolate. Allow to sit for 10 minutes; gently stir until smooth.
Cool to room temperature, stirring occasionally. Place the chocolate mixture in the bowl of an electric mixer fitted with the whisk attachment. Whip on medium speed until ganache holds its shape and is slightly lightened in color.