Place the 2 cups heavy cream in a large pot over medium heat and heat it until it slightly bubbles around the edges (you don't want to boil it). Transfer the cream to a large bowl, and add the 12 ounces semisweet chocolate. Let it stand for 5 minutes to melt the chocolate, then whisk until completely smooth.
Chill the ganache in the fridge for up to 1 hour, until slightly firm and cool. Using an electric mixer, whip the ganache (similar to making whipped cream) until reaching your desired consistency (shorter for loose whipped ganache, longer for firmer ganache).
Notes
Make sure the chocolate fully melts. This is THE most important step in this recipe. Let the chocolate and cream sit together for at least 5 minutes before whisking. This will ensure the chocolate is melted fully and creates an ultra-smooth frosting.
Use fresh cream. Because the ingredients list is short, please use good-quality ones! This means cream that's fresh, preferably not too close to its sell-by date.
Chill. If you have loads of time, you can let the cream and chocolate mixture cool on the counter to room temperature, then whip it. But, I find that it whips more quickly if it's been chilled in the fridge first - and I never have enough time to let it naturally cool! So, pop it in the fridge to speed up the process.
Use it fresh. You can store the ganache frosting for a couple of days in the fridge, but it will change in consistency. Because this is such an easy recipe, I always make a batch fresh and use it right away.
Different chocolate. Feel free to use this method with other chocolate varieties, such as milk chocolate or white chocolate.
Dairy-free: Use coconut cream for the cream and non-dairy chocolate for the chocolate.