Finely chop the dark chocolate and place it in a heatproof bowl.
In a saucepan, heat the heavy cream over medium heat until it starts to simmer.
Pour the hot cream over the chopped chocolate and let it sit for 2-3 minutes.
Gently stir the mixture until smooth.
Add the unsalted butter if desired and stir until melted.
Allow the ganache to cool at room temperature for about 30 minutes.
Use immediately for a pourable consistency or refrigerate for a thicker texture.
Whip the cooled ganache for a lighter texture if desired.
Use the ganache for glazing, filling, or frosting desserts.