Use a serrated knife to chop the chocolate into small pieces. Put the chocolate in a heat-proof bowl (with butter, if using) and set aside.
Warm the heavy cream in a heavy-bottomed saucepan to just simmering. Just small bubbles, not boiling.
Pour the cream onto the chocolate, cover and let sit for about 2 to 3 minutes, then start stirring. At first it will look grainy, but keep stirring until the mixture is smooth. Add any flavorings (if using) and stir again.
The ganache is ready to use as a drizzled frosting after cooling slightly, or as a filling when chilled and solid.
For a ganache frosting, allow the ganache to cool to room temperature. Use an immersion blender or a hand mixer to whip the ganache until it's thick, yet spreadable and lightened in color, about 4 to 5 minutes.
Choose your chocolate type: milk, semi sweet, or dark, depending on your preference. Just make sure it's good quality.Ganache Ratios (Adapted from Baker Bettie): Ganache is just generally equal parts (by weight) of chocolate and heavy cream, but the final consistency can be altered based on the ratio of chocolate to cream. Using a kitchen scale makes it easier to scale the recipe up or down as needed.
2 parts chocolate to 1 part heavy cream: cools to a very thick almost fudge-like mixture, and is great for truffles.
1 part chocolate to 1 part heavy cream: cools to a pudding-like texture which is useful for fillings, frostings, and thick glazes.
1 part chocolate to 1½ heavy cream: cools to a liquid thin enough to pour as a glaze and is thin enough to drink
There are 3 extra ingredients you can add if you'd like:
1 to 2 tablespoons of butter to add even more richness. It firms up the ganache if you're using it for making truffles.
½ teaspoon of flavorings after the mixture has been stirred completely. Try extracts like vanilla, mint, or Fiori di Sicilia (vanilla orange), or add a 1 tablespoon of bourbon, rum, or Kahlua for a boozy kick.
1 to 2 teaspoons of corn syrup to add shine to the ganache. Add it in with the vanilla.